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■DECLASSTFTPn 
Ey authority Secretary of 

SEP 2 6 1960 

Defense m emo 2 A ugust 1960 
LIBRARY OP CONGRESS 


SUMMARY TECHNICAL REPORT 
OF THE 

NATIONAL DEFENSE RESEARCH COMMITTEE 


This document contains information affecting the national defense of the 
United States within the meaning of the Espionage Act, 50 U. S. C., 31 and 32, 
as amended. Its transmission or the revelation of its contents in any manner 
to an unauthorized person is prohibited by law. 

This volume is classified^BmHI^fe in accordance with security regula- 
tions of the War an^NavyDenartments because certain chapters contain 
material which was^^l^MMMBfe at the date of printing. Other chapters 
may have had a lower classification or none. The reader is advised to consult 
the War and Navy agencies listed on the reverse of this page for the current 
classification of any material. 


CLAS SIFICATION CH ANGED TO 
AUTHORITY id) lose/ 


Manuscript and illustrations for this volume were prepared for 
publication by the Summary Reports Group of the Columbia 
University Division of War Research under contract OEMsr-1131 
with the Office of Scientific Research and Development. This vol- 
ume was printed and bound by the Columbia University Press. 

Distribution of the Summary Technical Report of NDRC has been 
made by the War and Navy Departments. Inquiries concerning the 
availability and distribution of the Summary Technical Report 
volumes and microfilmed and other reference material should be 
addressed to the War Department Library, Room 1A-522, The 
Pentagon, Washington 25, D. C., or to the Office of Naval Re- 
search, Navy Department, Attention: Reports and Documents 
Section, Washington 25, D. C. 


Copy No. 

22 % 



SUMMARY TECHNICAL REPORT OF DIVISION 14, NDRC 

.declassified 

VOLUME 2 By aUthority Secretary 0 f 

SEP 2 6 1960 


Defense memo 2 August 1960 

MILITARY AIRBOffftf]P ONGRESS 
RADAR SYSTEMS 
^ [MARS]> 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 

VANNEVAR BUSH, DIRECTOR 

NATIONAL DEFENSE RESEARCH COMMITTEE 
JA[MES B. CONAN T, CHAIRMAN 

DIVISION 14 
A. L. LOOMIS, CHIEF 


WASHINGTON, D. C., 1946 




NATIONAL DEFENSE RESEARCH COMMITTEE 


James B. Conan t, Chairman 
Richard C. Tolman, Vice Chairman 
Roger Adams Army Representative 1 

Frank B. Jewett Navy Representative 2 

Karl T. Compton Commissioner of Patents 3 

Irvin Stewart, Executive Secretary 


1 Army representatives in order of service: 

Maj. Gen. G. V. Strong Col. L. A. Denson 

Maj. Gen. R. C. Moore Col. P. R. Faymonville 

Maj. Gen. C. C. Williams Brig- Gen. E. A. Regnier 

Brig. Gen. W. A. Wood, Jr. Col. M. M. Irvine 

Col. E. A. Routheau 


2 Navy representatives in order of service: 

Rear Adm. H. G. Bowen Rear Adm. J. A. Furer 

Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 

Commodore H. A. Schade 
3 Commissioners of Patents in order of service: 

Conway P. Coe Casper W. Ooms 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suitable 
projects and research programs on the instrumentalities of 
warfare, together with contract facilities for carrying out 
these projects and programs, and (2) to administer the tech- 
nical and scientific work of the contracts. More specifically, 
NDRC functioned by initiating research projects on re- 
quests from the Army or the Navy, or on requests from an 
allied government transmitted through the Liaison Office 
of OSRD, or on its own considered initiative as a result of 
the experience of its members. Proposals prepared by the 
Division, Panel, or Committee for research contracts for 
performance of the work involved in such projects were 
first reviewed by NDRC, and if approved, recommended to 
the Director of OSRD. Upon approval of a proposal by the 
Director, a contract permitting maximum flexibility of 
scientific effort was arranged. The business aspects of the 
contract, including such matters as materials, clearances, 
vouchers, patents, priorities, legal matters, and administra- 
tion of patent matters were handled by the Executive Sec- 
retary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 
These were: 

Division A — Armor and Ordnance 

Division B — Bombs, Fuels, Gases, & Chemical Problems 
Division C — Communication and Transportation 
Division D — Detection, Controls, and Instruments 
Division E — Patents and Inventions 


In a reorganization in the fall of 1942, twenty-three ad- 
ministrative divisions, panels, or committees were created, 
each with a chief selected on the basis of his outstanding 
work in the particular field. The NDRC members then be- 
came a reviewing and advisory group to the Director of 
OSRD. The final organization was as follows: 

Division 1 — Ballistic Research 

Division 2 — Effects of Impact and Explosion 

Division 3 — Rocket Ordnance 

Division 4 — Ordnance Accessories 

Division 5 — New Missiles 

Division 6 — Sub-Surface Warfare 

Division 7 — Fire Control 

Division 8 — Explosives 

Division 9 — Chemistry 

Division 10 — Absorbents and Aerosols 

Division 11 — Chemical Engineering 

Division 12 — Transportation 

Division 13 — Electrical Communication 

Division 14 — Radar 

Division 15 — Radio Coordination 

Division 16 — Optics and Camouflage 

Division 17 — Physics 

Division 18 — War Metallurgy 

Division 19 — Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 


iv 



Library of Congress 



2015 


490939 


NDRC FOREWORD 


As events of the years preceding 1940 revealed more 
iJL and more clearly the seriousness of the world 
situation, many scientists in this country came to 
realize the need of organizing scientific research for 
service in a national emergency. Recommendations 
which they made to the White House were given care- 
ful and sympathetic attention, and as a result the 
National Defense Research Committee [NDRC] was 
formed by Executive Order of the President in the 
summer of 1940. The members of NDRC, appointed 
by the President, were instructed to supplement the 
work of the Army and the Navy in the development 
of the instrumentalities of war. A year later, upon the 
establishment of the Office of Scientific Research and 
Development [OSRD], NDRC became one of its 
units. 

The Summary Technical Report of NDRC is a con- 
scientious effort on the part of NDRC to summarize 
and evaluate its work and to present it in a useful and 
permanent form. It comprises some seventy volumes 
broken into groups corresponding to the NDRC Divi- 
sions, Panels, and Committees. 

The Summary Technical Report of each Division, 
Panel, or Committee is an integral survey of the work 
of that group. The first volume of each group’s re- 
port contains a summary of the report, stating the 
problems presented and the philosophy of attacking 
them, and. summarizing the results of the research, de- 
velopment, and training activities undertaken. Some 
volumes may be “state of the art” treatises covering 
subjects to which various research groups have con- 
tributed information. Others may contain descrip- 
tions of devices developed in the laboratories. A mas- 
ter index of all these divisional, panel, and committee 
reports which together constitute the Summary Tech- 
nical Report of NDRC is contained in a separate vol- 
ume, which also includes the index of a microfilm 
record of pertinent technical laboratory reports and 
reference material. 

Some of the NDRC-sponsored researches which 
have been declassified by the end of 1945 were of suffi- 
cient popular interest that it was found desirable to 
report them in the form of monographs, such as the 
series on radar by Division 14 and the monograph on 
sampling inspection by the Applied Mathematics 
Panel. Since the material treated in them is not dupli- 


cated in the Summary Technical Report of NDRC, 
the monographs are an important part of the story of 
these aspects of NDRC research. 

In contrast to the information on radar, which is of 
widespread interest and much of which is released to 
the public, the research on subsurface warfare is 
largely classified and is of general interest to a more 
restricted group. As a consequence, the report of Divi- 
sion 6 is found almost entirely in its Summary Tech- 
nical Report, which runs to over twenty volumes. The 
extent of the work of a Division cannot therefore be 
judged solely by the number of volumes devoted to it 
in the Summary Technical Report of NDRC: ac- 
count must be taken of the monographs and available 
reports published elsewhere. 

To A. L. Loomis, Chief of Division 14, the men 
who worked under his direction, and the personnel of 
the Division’s contractors belongs major credit for 
the perfection of a device which forcefully altered the 
course of the war. The application of radar by all 
Services in all theaters of operation is an eloquent 
testimonial not only to the skill of these men but also 
to their will, their loyal cooperation, and their scien- 
tific integrity. The Summary Technical Report of the 
Division, prepared under the direction of the Divi- 
sion Chief and authorized by him for publication, 
therefore not only describes a major portion of their 
technical activities but is also a record of able Ameri- 
can scientists and engineers cooperating fully in the 
defense of their country. 

It is assuring to know that their contributions in 
the new field of microwaves will not be placed in in- 
tellectual cold storage to await purely military appli- 
cations, but instead will soon find use in the industry, 
the transportation, the communications, and the 
scientific researches of a peacetime world. 

For their work in opening a broad entrance to a 
new field of knowledge as well as for their invaluable 
contributions in a time of desperate strife, we join the 
Nation in expressing our sincere appreciation. 

Vannevar Bush, Director 
Office of Scientific Research and Development 

J. B. Conant, Chairman 
National Defense Research Committee 



^ & / G^o j 































































































































Ui 1 1 Mr ■■ 





















FOREWORD 


T he first of the three projects initiated by the 
Microwave Section of the National Defense Re- 
search Committee in the summer of 1940 was the 
development of aircraft interception radar [AI] to 
permit successful attack of bombers by fighter planes 
at night or in overcast. Throughout World War II 
the development of radar equipment for military 
and naval aircraft was the major activity of Division 
14. In addition to the laboratory development of 
radar equipment, Division 14 was directly concerned 
with the several other steps necessary for the 
introduction of new equipment to Service use, in- 
cluding experimental trial, engineering, consultation 
in the production on Service contract, acceptance 
and operational tests, installation as well as main- 
tenance, modification, and other aspects of field 
service. It is logical, therefore, that in the Summary 
Technical Report of the Division program, one com- 
plete volume should be devoted to airborne radar. 

The Division 14 Summary Technical Report con- 
sists of three volumes of which MARS is the second. 
The first volume, Radar, contains a summary of the 
Division 14 and Radiation Laboratory activities and 
special project reports. It will serve as a general guide 
to the Division projects and activities. The third 
volume is a complete bibliography of Division 14 and 
Radiation Laboratory reports listed by serial num- 
ber, subject, organization, and in the case of Radia- 
tion Laboratory reports, by author. 

The largest publication effort of Division 14 is 
the Radiation Laboratory Series published by the 
McGraw Hill Book Company, which is included 
as a supplement to the Summary Technical Report. 
This series consists of some twenty-eight volumes 
and an index. It is a complete report on the state of 
the radar art at the end of World War II, including 
texts on fundamental electronics, component and 
system design and engineering, peacetime applica- 
tions, and Loran navigation. 

A brief review of all the component and systems 
projects undertaken within Division 14 is given in 
the Final Project Report dated December 1945. The 
history of Division 14 and the Radiation Laboratory 
is told in the book Radar, published by Little, Brown 
& Company as a part of the Long History of OSRD. 

The MARS volume was prepared for Division 14 
by the Massachusetts Institute of Technology Radia- 
tion Laboratory. The Radiation Laboratory as the 
principal contract of Division 14 accounted for over 


75 per cent of the Division’s dollar appropriations to 
contracts and participated in every phase of the 
Division program. 

MARS may therefore quite naturally emphasize 
Radiation Laboratory airborne radar developments, 
its policies, and its opinions. It is not, however, 
limited to a discussion of Radiation Laboratory 
activities. Wherever desirable for a complete treat- 
ment of the subject, use has been made of material 
from the industrial concerns which participated in 
the development and production of the equipments 
on Army and Navy as well as on OSRD contracts. 
Also included is material received from Service 
laboratories, testing agencies, and operational groups, 
including principally the results of their experience 
in the planning, development, and introduction of 
new equipment. It has not been possible in most 
cases to make direct reference to the sources of 
material. The assistance of these many organizations 
which took part in the wartime radar program and 
contributed directly or indirectly to the prepara- 
tion of this book is gratefully acknowledged. Specific 
acknowledgment is due to Bell Telephone Labora- 
tories, Inc., General Electric Company, Galvin Man- 
ufacturing Corporation, Philco Corporation, Lukas 
Harold Company, and Radio Corporation of America. 

MARS was written by members of the Airborne 
Division of the Radiation Laboratory with an Edi- 
torial Board consisting of F. R. Banks, J. J. Hibbert, 
L. J. Laslett, E. M. Lyman, Sims McGrath, R. M. 
Robertson, R. M. Thrall, and R. B. Whitney. Dr. 
Thrall was editor of MARS. He succeeded Dr. 
Laslett who as the first editor was responsible for the 
early planning and organization. It should be noted 
that the volume was written, after a short planning 
period, in less than four months. At this speed there 
was not sufficient time to permit all of the editing 
desirable. The individual authors of the various 
chapters were permitted complete freedom of opinion 
on controversial subjects such as the selection of 
equipment for future requirements as well as the 
most effective methods of test, introduction, and use. 

A. L. Loomis 
Chief, Division 14 

L. A. DuBridge 
Director, Radiation Laboratory 


TITLES OF DIVISION 14 SUMMARY TECHNICAL REPORTS 


SUMMARY TECHNICAL REPORT OF DIVISION 14, NDRC 

Volume 1 Radar: Summary Reports and Harp Project 
Volume 2 Military Airborne Radar Systems (MARS) 

Volume 3 Bibliography of Division 14 and Radiation Laboratory Reports 

RADIATION LABORATORY SERIES 
(Published by the McGraw-Hill Book Company) 

1. Radar System Engineering, Louis N. Ridenour 

2. Radar Aids to Navigation, J. S. Hall 

3. Radar Beacons, A. Roberts 

4. Loran, J. A. Pierce, A. A. McKenzie, R. H. Woodward 

5. Pulse Generators, G. N. Glasoe, J. V. Lebacqz 

6. Microwave Magnetrons, George B. Collins 

7. Klystrons and Microwave Triodes, D. R . Hamilton, J. K. Knipp, J. B. H. Kuper 

8. Principles of Microwave Circuits, C. G. Montgomery, E. M. Purcell, R. H. Dicke 

9. Microwave Transmission Circuits, G. L. Ragan 

10. Waveguide Handbook, N. Marcuvitz 

11. Technique of Microwave Measurements, C. G. Montgomery 

12. Microwave Antenna Theory and Design, S. Silver 

13. Propagation of Short Radio Waves, D. E. Kerr 

14. Microwave Duplexers, L. D. Smullin, C. G. Montgomery 

15. Crystal Rectifiers, H. C. Torrey, C. A. Whitmer 

16. Microwave Mixers, R. V. Pound 

17. Components Handbook, John F. Blackburn 

18. Vacuum Tube Amplifiers, George E. Valley, Jr., Henry Wallman 

19. Waveforms, Britton Chance, F. C. Williams, V. W. Hughes, D. Sayre, E. F. MacNichol, Jr. 

20. Electronic Time Measurements, Britton Chance, R. I. Hulsizer, E. F. MacNichol, Jr. 

21. Electronic Instruments, I. A. Greenwood, Jr., D. MacRae, Jr., H. J. Reed, J. V. Holdam, Jr. 

22. Cathode Ray Tube Displays, J. T. Soller, M. A. Starr, George E. Valley, Jr. 

23. Microwave Receivers, S. N. VanVoorhis 

24. Threshold Signals, J. L. Lawson, G. E. Uhlenbeck 

25. Theory of Servomechanisms, H. M. James, N. B. Nichols, R. S. Phillips 

26. Radar Scanners and Radomes, W. M. Cady, M. B. Karelitz, L. A. Turner 

27. Computing Mechanisms and Linkages, A. Svoboda 

28. Index 

viii 



PREFACE ? 


T he primary purpose in writing the book, Mili- 
tary Airborne Radar Systems [MARS], was to 
put in permanent form a description of the status of 
radar for military use in airplanes, at the close of 
World War II. The book MARS consists of 24 chap- 
ters of which the first is general and the remaining 23 
are divided into five parts. These parts represent a 
subdivision of military airborne radar systems into 
classes according to function. 

Part I treats aircraft-to-surface vessel [ASV] radars. 
ASV comes first partly because of its historical posi- 
tion among radar systems but primarily because of 
the relative simplicity of the ASV systems. It is hoped 
that this simplicity will give the reader who is un- 
familiar with radar systems an easy approach to the 
subject. The titles of the four chapters that comprise 
Part I indicate the major considerations which apply 
to all radar systems. These considerations are: 
(1) function of the system, namely, what the system 
is to be used for; (2) the theoretical basis for and 
general features of the system; (3) what things must 
be kept in mind in the actual design of the system 
once its general features have been decided; (4) how 
the system can be made to work and kept working 
and how well it does work. 

Part II treats radar bombing. The opening chap- 
ter, Chapter 6, contains a summary of the part and a 
brief introduction to the geometric problem of bomb- 
ing including a partial description of the theory and 
operation of the Norden bombsight. Chapter 7 treats 
the airborne radar systems used for bombing. Chap- 
ters 8 and 9 treat bombing computers to be used with 
these airborne radar systems. Not all bombing com- 
puters are discussed but enough are treated to give 
a fair picture of the state of the art at the close of 
World War II. Chapters 10 and 11 discuss ground- 
aided bombing — beacon bombing in Chapter 10 and 
close-support bombing in Chapter 11. Chapter 12 
treats toss bombing, a type of bombing used by 
fighter planes and light bombers. Problems of toss 
bombing are almost completely unrelated to those of 
the other kinds of bombing treated in the earlier 
chapters, being more closely related to the problems 
of fire control (see Part IV). Chapter 13 treats 
problems of assessment and training incidental to 
the successful use of radar for bombing. 

Part III treats aircraft interception [AI]. Chapter 
14 introduces the problem; Chapter 15 discusses the 
equipments used; and Chapter 16 deals with AI 
tactics. 


Part IV treats airborne radar systems used for fire 
control. Chapter 17 is introductory in nature and 
emphasizes some of the problems encountered in the 
development of airborne radar fire-control systems. 
Chapter 18, “Automatic Following Equipment,” 
treats the most complicated of the fire-control sys- 
tems. Chapter 19 treats a somewhat less complicated 
(nonautomatic) type of equipment. Chapter 20 treats 
radar systems used for range-to-target only. This in- 
cludes systems suitable for ranging from one aircraft 
to another aircraft, systems suitable for ranging from 
an aircraft to a ground target, and systems which 
will do both. Chapter 21 treats computers (gunsights) 
to accompany the radar systems. Since very little of 
the computer work was done at the Radiation 
Laboratory [RL] there is very little discussion of 
specific computers in Chapter 21. The orientation of 
the chapter is toward the problems of combining 
radar systems and computing systems into overall 
fire-control systems. (Other volumes of the STR give 
quite satisfactory coverage and descriptions of spe- 
cific computers. See the bibliography of Part IV.) 
Chapter 22 discusses the problems of assessing fire- 
control radar systems and is based on experiences of 
RL personnel, both at Service testing agencies and 
at the Radiation Laboratory. 

Part V, airborne moving target indication [AMTI], 
treats a series of developments which came at the 
very end of the war. These systems show considerable 
military promise, but the treatment of them in the 
present volume is confined primarily to the technical 
details of the system. 

The foregoing description of the various parts of 
book MARS does not give a complete picture of the 
purpose of the volume. We accordingly supplement 
the above part-by-part description with a discussion 
of the several theses of the volume and the places 
where these are treated. The basic purpose of the 
book MARS , as the general title Summary Technical 
Report implies, is to give a description of the status 
of airborne radar at the close of World War II. 
Realizing that the rapid rate of development of radar 
systems will make most of the systems in use during 
the war obsolete (in some cases even by the time this 
book is released) only a minimum number of systems 
have been described in detail. The reader is referred 
to the bibliography for detailed technical descrip- 
tions. The chapters which are primarily concerned 
with description of radar systems are 3, 4, 5, 7, 15, 
18, 19, 20, 23, and 24. 


IX 


PREFACE 


In many cases effective use of a radar system re- 
quires auxiliary computers. Given a radar system 
and a computer to go with it, it is then necessary to 
study the tactical importance of the combination. 
Discussions of tactics and computers come in Chapter 
2 for ASV and in Chapters 8, 9, 10, 11, and 12 for 
bombing, in Chapters 14 and 16 for AI, and in 
Chapter 21 for fire control. 

After a radar system and the appropriate attach- 
ments have been developed it becomes important to 
consider how the combination will function. It may 
be that several systems all designed for the same 
general function are offered to the Armed Services at 
approximately the same time. It is then necessary 
for the military testing agencies to conduct compara- 
tive performance tests so as to decide which of the 
competing systems should be developed. The im- 
portance of assessment is emphasized in Chapter 1; 
Chapter 22 gives a picture of the assessment situa- 
tion for fire-control systems. In addition to assessing 
the systems, there is the major problem of assessing 
performance of military personnel being trained to 
use the equipment; this is discussed in Chapter 13, 
which also treats the general importance of an 
effective training program not only for operators but 
for maintenance men and staff officers as well. The 
problem of maintenance of radar systems as it con- 
cerns test equipment needed and procedures to be 
followed is discussed first (and in most detail) in 
Chapter 5 for ASV. Certain modifications for the 
more complicated radar systems are considered in 
Chapter 7 for bombing and Chapter 15 for AI. 

Few of the staff members of the Radiation Labora- 
tory realized when they entered the field of military 
research how long a road must be traversed in order 
to successfully introduce a new piece of equipment 
into military use. The initial thought of many a 
scientist was that a sufficient contribution would be 
to supply a laboratory model of the proposed new 
equipment. He realized that he might have to show 
the equipment to a representative of the Armed 
Services and later, perhaps, spend a few weeks with 
the engineer and the manufacturer for the equip- 
ment. In contrast to this early expectation, it was 


soon learned that the man who designed the equip- 
ment should follow it closely every step of the way. 
This includes, first, demonstrating the merits of the 
equipment to the Armed Services, which usually 
entails close cooperation with the military testing 
agencies; next, the designer must keep in close touch 
with the manufacturer and the Armed Services 
throughout the whole preproduction stage; he should 
follow his equipment into the training stages and 
then finally to the combat theater. The extent of 
participation by Radiation Laboratory personnel in 
these various phases is indicated in the summary 
report of Dr. L. A. DuBridge (see Volume 1 of 
the Division 14 STR). The chapters of the book 
MARS which treat the general position of the 
scientist in warfare and related problems are 1, 13, 
17, 21, and 22. 

The reader’s attention is called to the presence of 
the bibliography, glossary, and index at the end of 
the book. The arrangement of the bibliography and 
the notation for references are discussed at the begin- 
ning of the bibliography. 

The table of contents indicates the author (or 
authors) of each section of the book. I wish to express 
my appreciation to all of the authors; to the members 
of the MARS editorial board, and especially to 
Dr. R. B. Whitney, who served as Associate Editor 
during the latter months of the preparation of this 
volume; and to the MARS production staff: Emily 
Blech, Doris Burton, Ann Klein, Jean McBeath, 
Pagonia Poulos, Jennie Scarponi, Bernice Zamett, 
for their cooperation and loyalty in the various 
stages of publication. 

Finally, Division 14 wishes to state its appreciation 
to the U. S. Army and to the following organizations 
for permitting the use of their material in the illus- 
tration of this volume : Bell Telephone Laboratories, 
Galvin Manufacturing Corporation, General Electric 
Company, and the Sperry Gyroscope Company. The 
source of the figures is the Radiation Laboratory un- 
less otherwise noted. 

R. M. Thrall 
Editor 



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This volume, like the seventy others of the Sum- 
mary Technical Report of NDRC, has been writ- 
ten, edited, and printed under great pressure. 
Inevitably there are errors which have slipped past 
Division readers and proofreaders. There may be 
errors of fact not known at time of printing. The 
author has not been able to follow through his 
writing to the final page proof. 

Please report errors to: 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D. C. 

A master errata sheet will be compiled from these 
reports and sent to recipients of the volume. Your 
help will make this book more useful to other 
readers and will be of great value in preparing any 
revisions. 


CONTENTS 


e 


CHAPTER PAGE 

1 Scientists in Warfare by M. G. White 1 

PART I 

RADAR MEANS FOR DETECTION OF 
SURFACE VESSELS FROM AIRCRAFT 

By S. McGrath 7 

2 Functions of ASV Radars by G. A. Garrett 9 

3 Means of ASY Detection by G. A. Garrett , S. McGrath. . 13 

4 Design Considerations by G. A. Garrett , S. McGrath . . 17 

5 ASV Performance by S. McGrath, A. D. Cole, G. R. Banks 25 

PART II 

RADAR BOMBING 

By J. J. Hibbert 35 

6 The Bombing Problem by W. J. Tull, S. McGrath ... 37 

7 Airborne Radar Systems for Bombing by J. J. Hibbert, 

D. Holliday, D. L. Hagler, F. R. Banks 47 

8 Bombing Computers for Radar Mapping Systems by W. J. 

Tull, D. L. Hagler, E. A. Luebke, J. J. Hibbert, W. Deerhake, 

S. McGrath 66 

9 The GPI Bombing and Navigation Computer by W. J. 

Tull, N. W. MacLean 87 

10 Beacon Bombing by J . B. Platt, R. B. Lawrance .... 104 

11 Ground-Controlled Bombing by E. Lyman, G. D. Huff . 131 

12 Application of Radar to Toss Bombing by L. J. Laslett . 139 

13 Radar Bombing Assessment and Training by J. J. Hibbert, 

G. E. Wheeler, J . E. Ward, T. A. Murrell, E. A. Luebke, 

V. Holdam,, J. H. Buck, E. M. Lyman 155 




xri 


CONTENTS 


PART III 

AIRCRAFT INTERCEPTION PAGE 

By R. McG. Robertson 173 

CHAPTER 

14 The Interception of Enemy Aircraft by R. McG . Robertson 175 

15 AI Equipment by W. M. Cady , F. R. Banks 178 

16 Aircraft Interception Tactics by F. R. Banks , T. W. Bonner 188 

PART IV 

AIRBORNE FIRE-CONTROL RADAR 

By R. B. Whitney 195 

• 

17 General Considerations on Airborne Fire Control by R. M. 

Thrall, R. B. Whitney 197 

18 Automatic Following Equipment by H. G. Brewer . . . 200 

19 Manually Directed Radar Gunsights by V. Holdam . . 219 

20 Combined Optical and Radar Fire Control by E. H. B . 

Bartelink 226 

21 Computer Problems by R. M. Thrall 248 

22 Assessment Problems by R. B. Whitney 256 

PART V 

AIRBORNE MOVING TARGET INDICATORS 

By E. M. Lyman 277 

23 Basic Principles of AMTI by E. M. Lyman 279 

24 Airborne Moving Target Indicator Systems by E. M. 

Lyman 287 

Glossary 303 

Bibliography 309 

OSRD Appointees 320 

Index 321 




Chapter 1 


SCIENTISTS IN WARFARE 


1.1 GENERAL PURPOSE OF BOOK 

This book on military applications of airborne 
radar systems is presented by members of the Air- 
borne Division of the Radiation Laboratory with the 
hope that it will prove interesting and stimulating 
to those who take up our work as we disband. Our 
point of view in writing about radar systems is quite 
different from that adopted in the Radiation Labora- 
tory Technical Series. a Where the latter series dis- 
cusses technical design in detail and is intended to lay 
bare the scientific framework of radar, this volume 
treats the subjects from the overall operational point 
of view. If one compares the Technical Series to re- 
ports on medical laboratory research it would be ap- 
propriate to classify this present volume as a clinical 
study of the “patient-as-a-whole.” Throughout this 
presentation we emphasize the tactical requirements, 
the military problem, the reaction of the operators 
and, generally speaking, all those diverse human ele- 
ments that only experience uncovers. 

A book which deals in specific equipments runs the 
risk of becoming dated even before it reaches the 
public. The reader is urged to consider the following 
chapters as more than a mere history of airborne mi- 
crowave radar from 1940 to 1945. They should be re- 
garded as a study of the application of new scientific 
and technological developments to problems of war- 
fare. Should war come again, some other powerful 
technical innovation may require the coordinated 
efforts of this country’s scientists, engineers, and mil- 
itary personnel. It is to be hoped that lessons learned 
in this war will not pass unnoticed and that the fum- 
bling in the dark days of 1940-1942 will not be re- 
peated. The reader should therefore pay close atten- 
tion to the tactical thinking, to the general approach 
to the military problem in order to gain insight into 
the factors which made possible the development of 
so many important radars. Although the equipments 
to be discussed may well be stepping stones to the 
future, we regard the scientific point of view which 
this book tries to convey as more important than de- 
tails of equipment. 

a A technical series of approximately thirty volumes has been 
prepared by the Radiation Laboratory J staff for publication 
by the McGraw-Hill Book Oo. 


1.2 SCIENTISTS AND MILITARY 
PROBLEMS 

Before turning to specific military problems, we 
may profitably discuss the general relation between 
scientists and military problems, for this relation al- 
most completely controls the degree to which the 
scientist can contribute. 

Even a casual study reveals that big advances in 
tactical thinking and technical warfare have come 
from sources but very loosely tied to the military 
agencies. By the very nature of a major advance it is 
impossible to predict or “ whistle up” a revolutionary 
discovery. This holds for straight tactical, technical 
thinking as well as for basic science. It is necessary, 
therefore, to avoid excessive regimentation (often in- 
troduced in the guise of coordination or standardiza- 
tion) lest available national talent be too sharply di- 
rected into fields of short-range interest. For this and 
many other reasons it is best to rely upon civilian 
scientists in civilian institutions for the major techni- 
cal and associated tactical advances. Obviously the 
detailed working out of tactical employment can only 
be done within the military structure, but even here 
World War II has demonstrated that civilian scien- 
tists belonging to civilian agencies must have the op- 
portunity to introduce new equipment and even new 
tactical concepts at all levels of the military organ- 
ization. Failure to recognize this function as legiti- 
mate has cost unnecessary delay, inefficient use, and 
even complete lack of appreciation of potentialities 
on the part of military agencies. 

A book such as this is intended to be cannot pre- 
sent all the evidence in support of this thesis, but if 
any convincing is required, a comparison may be 
made between the wartime role of our American sci- 
entists and that of the German and Italian. Anglo- 
American science was essential to winning the war. 
Axis scientists were quickly surpassed by the victors, 
largely because the latter scientists were generally 
welcomed by the military at all levels even though 
they were associated with a separate organization in 
which they were free to work on new developments 
no matter how unpromising they appeared initially. 
There is little need to fear that free scientists, prop- 
erly informed, will ignore the problems clamoring for 


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SCIENTISTS IN WARFARE 


attention; indeed there is more danger that they will 
be overly impressed by the current military prob- 
lems and so fail to make revolutionary developments 
in advance of their need in battle. 

According to all information available to us the 
Axis’ use of scientists was in sharp contrast to our 
policy. Second-rate scientists climbed into positions 
of authority and never gained the full confidence of 
the military, with the result that the scientists, being 
uninformed, indulged in many useless and fantastic 
enterprises. Exceptions, such as rocket development, 
do not alter the general belief that German scientists 
found little encouragement to consider themselves 
essential to the winning of the war. This was indeed 
most fortunate for us, as we now know after the 
events at Hiroshima and Nagasaki. 

Axis scientists suffered either from over direction 
in government and industrial laboratories, or else had 
little contact with the military problem. True, the 
military agencies granted contracts to various insti- 
tutional laboratories, but apparently very little of 
direct military interest was expected or delivered. 
This is in strong contrast with the way in which the 
Office of Scientific Research and Development or- 
ganized and directed the work of American scientists. 

1.3 THE SECURITY PROBLEM 

As was pointed out, science in its military appli- 
cations as well as in its basic form, must be a “free 
science” in order to be strong. If this is granted, sev- 
eral more steps should be taken by both military and 
scientific agencies. One has to do with the thorny 
question of secrecy. Contributing parties must be 
adequately informed about the tactical and technical 
problems. In spite of this obvious fact, there was far 
too much indiscriminate, blind classification of mili- 
tary information, scientific discoveries, technical 
equipment, and correspondence. Not only were our 
civilian scientists given too little access to military 
planning but they were also kept in mutual ignorance 
of scientific advances in cognate fields. Discoveries 
made in radar should have received much wider dis- 
semination to those working in communications, tele- 
vision, underwater sound, and other fields. That these 
discoveries were not so distributed is a sad reflection 
on the scientists themselves who were temporarily 
forgetful of the very essence of creative thinking — 
freedom of publication. No one is suggesting unre- 
stricted publication in the public journals, but surely 
there could have been a series of classified journals, 


available to all cleared scientists, which would have 
broken down artificial and highly injurious barriers. 
The writer has personal knowledge of many instances 
where greater restricted distribution of basic scien- 
tific and technological data would have profoundly 
increased our scientific strength. 

With the coming of peace and talk about split- 
second pushbutton warfare of the future, we are faced 
with a most difficult problem in secrecy. In the midst 
of war it is clear that the best security lies in speed, in 
achievement, and not necessarily in secrecy. That 
secrecy can defeat its own purpose is shown by the 
frequency with which enemy scientists independently 
discovered techniques zealously guarded by us. Our 
secrecy merely slowed our own production and de- 
creased our time advantage. A classical example is 
the case of the German Air Force which withheld in- 
formation on captured British microwave bombing 
equipment from the German submarine command. 
Six precious months were thus lost by the Germans in 
combating microwave aircraft to surface vessel [ASV] 
radar. Perhaps the whole war and history of the 
world was affected by this instance of the shortsight- 
edness of excessive secrecy. 

Though wartime secrecy concerning engineering 
advances usually serves to keep the wrong people in 
ignorance, we must concede that peacetime secrecy is 
more of a problem. Here the element of time and 
speed is lacking; so if we assume that a truly impor- 
tant weapon can be kept secret there is point in limit- 
ing complete information to a few people only. Of 
course, such limitation will automatically retard tech- 
nical progress, and perhaps retard quantity produc- 
tion in the event of war. But how can we assume that 
we can keep a secret, when in fact it is exceedingly 
difficult to find any important weapon or scientific 
advance that was not the contemporaneous interest 
of potential belligerents? This generally universal de- 
velopment of “secret” weapons should surprise no 
one, for creative thinking does not come as a bolt 
from the blue. It comes from the free interchange of 
ideas between all manner of people doing all manner 
of work. Modern radar owes much to virtually all 
nationalities working for hundreds of years. The 
atomic bomb roster includes Germans, Italians, 
French, English, Americans, Canadians, and Austri- 
ans, to mention but a few nationalities. 

Before the war England, Germany, Japan, France, 
Italy, and the United States were all closely guarding 
radar as a highly secret device. By such close control 
our progress was immeasurably hampered, but to be 


SELECTION AND TRAINING OF SCIENTIFIC PERSONNEL 


3 


sure the enemy also probably failed to benefit from 
our discoveries. At the start of this war our radar was 
far behind what it could have been, but Japanese 
radar was still farther behind and that of the Ger- 
mans somewhat ahead of ours. It is likely that our 
radar advantage over the Japanese would have been 
reduced by free publication, while it is interesting to 
speculate on how soon the whole show would have 
been given away anyhow under the impact of normal 
peacetime demands for the facilities which radar of- 
fers. The best one can hope for, even with tight se- 
curity control, is a slight time advantage of a few 
months to a year or so and there is a very good chance, 
that excessive secrecy, by slowing down production, 
will actually lose any advantage gained by prior in- 
vention. 

We can only conclude that in the past, and in- 
creasingly so in the future, no nation has had or will 
have a monopoly on scientific advance. No way is 
known to keep secret a fact of nature; it is present for 
all inquiring minds to discover and apply. Of nearly 
equally dubious validity is the belief that engineering 
“know-how” can be kept secret to the advantage of 
the possessor. Only rarely is an important weapon 
significant in small quantities, and if it requires mass 
production to be useful it must be designed with the 
national technical structure in mind. Airplanes, rock- 
ets, radars, guns, ships, tanks, radios are a clear re- 
flection of our industrial life and as such hold few real 
secrets. In fact, our thinking is distressingly influ- 
enced by what has been done before, so that under 
the stress of war we turn again and again to estab- 
lished practice. 

Wherein, then, does security lie? Some will argue 
that large standing armies and full military prepared- 
ness are the only safeguard. Others believe in a small, 
highly trained army of mechanical wizards possessed 
of an array of push buttons marked with the names of 
potential enemies. We even hear the attack-now the- 
ory which urges us to use our temporary alleged su- 
periority in atomic bombs to wipe out anyone and 
everyone who might one day wipe us out. 

To suggest that all this is unconstructive, hysteri- 
cal, and even insane is perhaps out of place in a book 
devoted to the “creative” side of war. But as we re- 
marked earlier, this volume attempts to approach 
warfare as a whole. By our intensive study of techni- 
cal warfare during the most destructive war of mod- 
ern times we, as scientists and engineers, are qualified 
to speak of more than boxes, knobs, rectifiers, and an- 
tennas. We have observed and reflected on the nature 


of technical warfare and can only conclude that no 
nation can, by scientific and industrial effort alone, 
insure itself against annihilation. There is no impreg- 
nable defense; there is no guarantee of victory no 
matter who starts another war. 

All we achieve by peacetime military effort is a 
little time in which to marshal our forces. War, large- 
scale war, will not strike without warning, without a 
period of mounting tension and expanding arma- 
ments. No nation will go to war without preparations 
plain for all to see. Pearl Harbor was no surprise; it 
was a question only of “when” and “where.” Before 
Pearl Harbor the scientists of the country were mo- 
bilizing and working at top speed. The Radiation 
Laboratory, organized in November 1940, had 
pushed far into microwaves before the Japanese 
struck and it was microwave radar on shipboard 
which turned the tide in the sea battles of early 1942. 
Our strength lay in a healthy scientific and engineer- 
ing tradition of education; the same tradition which 
has created our entire industrial life. In any future 
war it will again be the trained engineers, scientists, 
industrialists, and administrators who will pull us 
through; if anyone is left to be pulled through! 

1.4 SELECTION AND TRAINING OF 
SCIENTIFIC PERSONNEL 

Coming full circle to our original intention in this 
chapter, which was to catalogue the essential organs 
in the military scientific body, we urge that primary 
attention in all Service laboratories, and in all the 
military structure, be given to the personnel and not 
to gadgets. It is futile to develop a host of guided mis- 
siles at the expense of the broad training of the minds 
of young engineers and scientists. During the five 
years of World War II we stopped broad training in 
physics, chemistry, and mathematics in order to ac- 
celerate our industrial output. Now, of all times, we 
find large programs for all types of weapons, which 
though they may be stepping stones to the future, 
will not contribute one iota to revolutionary advance. 
Young men who should be encouraged to take ad- 
vanced studies in basic science are tinkering with 
more of the same gadgets which they worked on in 
time of war. Peacetime is the time for advance in 
basic science, for normal industrial pursuits, and for 
general education. What we must do now is revive a 
strong interest in the individual, in the broad training 
of his mind, in a rewakening of his sense of balance. 
Our success in waging a relatively bloodless, mecha- 


4 


SCIENTISTS IN WARFARE 


nized war has misled many into placing undue peace- 
time emphasis on the design of equipment rather than 
on the training of mind, and uncovering new facts of 
nature. 

Military laboratories, and all those engaged in mili- 
tary research wherever they may work, must broaden 
their intellectual base by holding seminars on general 
science, by inviting other scientists and engineers to 
participate in their problems, and by actual attend- 
ance at neighboring universities. Perhaps eminent 
scientists, both industrial and academic, can be in- 
duced to accept part-time consulting jobs. Going one 
step farther, it should be possible to attract capable 
university scientists to work in military laboratories 
for short periods; particularly if rare or costly equip- 
ment is thereby made available to them. Of course, 
freedom to publish facts-of-nature must be fully as- 
sured before capable scientists will become interested. 
What the military laboratory gets from this generous 
attitude is contact with a fresh and perhaps stimulat- 
ing mind. Whatever else it gets in the form of tangi- 
ble results is an extra dividend. 

We have dealt at length with the problem of get- 
ting scientific ability and outlook into the military 
establishments, and we have implied that always 
there will be many more scientists outside the pale 
than in, and desirably so. The country as a whole is a 
vast scientific reservoir which must be understood by 
the military if it is to be drawn upon. Defense of 
country is every citizen’s responsibility and preroga- 
tive. There must be no resentment if a civilian scien- 
tist exercises that prerogative and tries to discover all 
he can about military needs. He is no more likely to 
sell out his country than a man in uniform; and he is 
often more sensitive to his secrecy obligations than is 
the military man who may not know the entire tech- 
nical story. 

Nor is it enough simply to tell the scientist what- 
ever he wants to know. One must go farther than this, 
for the scientist does not know in advance what he 
should be told in order to arrive at a new weapon as 
yet not conceived. The only way known to get re- 
sults is for the military liaison officers to help the sci- 
entist live a vicarious combat existence in all details 
except actually slogging through the mud. Even the 
latter is sometimes stimulating, though generally the 
would-be inventor comes out with the conviction that 
what is most needed is more rugged equipment of the 
existing types. Since this is just the point of view of 
many military men, there is little to be gained by 
urging the scientist to get too close to his problem. A 


certain degree of detachment has always been one 
prerequisite to a major advance. 

Even so, the military agencies must avoid detach- 
ing the civilian scientists too completely from the ex- 
perimental laboratory of war. There existed a strong 
tendency for military men to shield the civilian labo- 
ratories from the tactical problems of war. Either by 
implication or direct statement the civilians are in- 
formed that logistics, strategy, and tactical maneuver 
are not to be understood by them. A great deal has 
been said, however, about the cold, the mud, the need 
for simplicity, and other self-evident truths. An ap- 
proach such as this is bound to induce the scientist 
merely to make improvements in existing equipment 
and to dull his sensibilities for more creative 
work. 

One last remark on the endless subject of free in- 
terchange concerns information on enemy develop- 
ments. Nearly everyone, in and out of military cir- 
cles, deplored the very strong tendency to treat cap- 
tured enemy secrets as “especially secret.” Why this 
should have been established policy is beyond logic, 
because no one gained by it except the enemy. Coun- 
termeasures which scientists could easily have de- 
veloped were slow to materialize because only the in- 
telligence officers were in possession of the enemy 
data. A policy of immediate and accurate reporting of 
all enemy technological and scientific advances would 
have helped this country’s research and perhaps led 
to more successful countermeasures. Even if it be 
argued that the “uncontrolled” scientists might have 
“let leak” to the enemy the fact that we knew of his 
activities, there would be little, if any harm done. In 
such an event, the enemy could either abandon his 
equipment — a gain for us — or somehow^ change 
its characteristics. To accomplish the latter in any 
useful and timely way is, in the case of radar at least, 
a very difficult engineering feat. Just the engineering 
manpower thereby diverted is, in itself, a serious 
drain on a country. 

It may be argued that disclosure of captured tech- 
nical data may jeopardize our counterintelligence 
service if the enemy becomes aware of our activity. 
A clear answer exists here because such disclosure 
would not be broadcast to the enemy; it would be 
made to the scientific laboratories already ac- 
customed to safeguarding their own secrets. The 
enemy’s first knowledge of a leak in his own secrecy 
screen would be the appearance of an attempted 
countermeasure. Since considerable delay would 
normalty exist between uncovering an enemy’s secret 




CIVILIAN SCIENTISTS AND MILITARY AGENCIES 


5 


and developing and applying a countermeasure, there 
would be a good chance for our counterintelligence 
agents to cover up. But in the last analysis one may 
well ask, “Why go to all the trouble to uncover the 
enemy’s technical secrets if no countermeasure is 
forthcoming? If valuable information is to be with- 
held from those who can effectively utilize it, why jeop- 
ardize the lives of our counterintelligence agents?” 

1.5 PROBLEMS OF COOPERATION 
BETWEEN CIVILIAN SCIENTISTS 
AND MILITARY AGENCIES 

We turn now to other problems of cooperation be- 
tween civilian scientists and the military agencies. 
Assuming that the civilian agency, be it industrial or 
educational, has devised some new scientific weapon, 
there arises the problem of demonstration and proof 
testing. Perhaps it was inescapable in the stress of 
war that our Armed Services were badly organized 
for testing new equipment quickly and intelligently. 
Certainly there was very little military personnel ad- 
equately trained for tactical testing of radar; for by 
the very nature of any new advance there will rarely 
be available personnel with sufficient detailed knowl- 
edge to assess a new weapon competently. 

One would think that this is so obvious as to call 
for a close working agreement between the military 
proving grounds and the external scientific agencies 
whence spring the new devices. On the contrary, the 
civilian was sometimes regarded with great distrust, 
not to say suspicion, of his motives in helping the 
testing agency. Charges of biased judgment and 
super-salesmanship only served to obscure the fact 
that the military testing agencies lacked the person- 
nel, equipment, and experience for keeping pace with 
technological warfare. There were some notable ex- 
ceptions to this rule. Toward the end of the war, after 
four years of striving, a definite improvement was in 
evidence. Can we not recognize the importance of 
welding all elements into one powerful team? Must 
we again take four years to develop scientific team- 
work? 

Some proving grounds encourage the scientist just 
to stay around and keep the equipment running; but 
rarely is the scientist encouraged to study the tactical 
employment of his instrument of war. Instead, this is 
the prerogative of junior officers with less technical 
experience and little background in battle. Naturally 
it is not easy to attract outstanding officers to prov- 
ing-ground work, for the glory is slight and advance- 


ment slow. In military circles there is no tradition of 
technical understanding and objective scholarship. 
Headlines go to colorful, dashing officers who take 
their technical weapons as a matter of course. 

But the country does not lack engineers and scien- 
tists able and eager to help keep our country strong. 
It is necessary, however, to develop the proper per- 
spective which will bring scientific training into con- 
tact with the technical problem of introducing new 
weapons. 

Going on now to the problems of combat use of 
equipment that has been hurried through the proving 
ground, we again find an important, but transient, 
role for the scientist and engineer. Training methods 
must be set up, and if the weapon is fairly radical, 
much valuable time can be saved if the inventing sci- 
entist will but give a little attention to assisting the 
training commands. Education is always slow, but if 
it is confined to the usual chain process where one 
man tells his neighbor and in turn the neighbor passes 
it on, there is a good chance for confusion and delay. 
This past war found civilians taking an active, if be- 
lated, part in training. Generally they were welcomed 
and found very helpful. 

A lesson learned slowly during the five years of 
radar introduction was the immense value of training 
aids and especially scoring devices. In bombing and 
fire control our hitting power would have been 
greatly increased if simple scoring methods had been 
more readily available. Imagine training a rifleman 
without some sort of score keeping, and yet that was 
virtually the situation in radar bombing and aerial 
gunnery. It was no exaggeration to call radar bomb- 
ing “blind bombing.” And yet, scoring methods were 
developed which, with adequate military backing, 
could have seen early and wide employment. Had 
these been pushed there is little doubt that our air 
force could have been several times more effective. 

It should be axiomatic that training, training aids, 
and scoring aids should receive early and competent 
attention. Nor should scoring aids be confined to the 
training phase alone. Training cannot duplicate ac- 
tual combat, and it is therefore essential to establish 
and to keep informed on the combat accuracy of any 
equipment, new or old. Combat accuracy evaluation 
will do much to keep up a man’s interest and morale 
and at the same time will give information of great 
value in planning operations. The generally increased 
feeling of responsibility in all quarters should materi- 
ally increase the effective striking force. In aerial gun- 
nery, only a small percentage of the bullets came 


RESTRICTED 


6 


SCIENTISTS IN WARFARE 


anywhere near the target so that even a small in- 
crease in accuracy should bring important results. 
Radar bombing was notably inaccurate and much 
worse than it needed to have been. Expressed in 
terms of concentration of bombs on the target we 
were only about 2 per cent as powerful as we could 
have been. Though accuracy expressed in this way is 
by no m6ans the whole story, and it would be unfair 
to charge that our Air Force could have been fifty 
times more powerful with the same cost to the coun- 
try, it is the considered opinion of many that better 
trained operators and combat scoring would have had 
a powerful effect on our bombing offensive. 

Along with scoring aids for combat evaluation it is 
imperative to have a team of analysts capable of in- 
terpreting combat results and making recommenda- 
tions for future employment. A subject as large and 
important as operations analysis cannot be profitably 
discussed here. We suggest that our war experience 
was only moderately successful in so far as radar was 
concerned and suffered from very poor liaison be- 
tween interested agencies. Too few radar experts were 
interested in analysis, and too few analysts knew any- 
thing about radar. 

A sweeping generalization of this nature cannot be 
dismissed by saying that war will always bring about 
a manpower crisis; our troubles were due largely to 
inadequate appreciation of the problem as a whole. 
Assuming that future wars will raise similar prob- 


lems we can hope that less time will be lost in getting 
everyone off on the same foot. 

In the chapters to follow the reader will find evi- 
dence of the strength and weakness of civilian scien- 
tific laboratories. In ways not easy to prove, but visi- 
ble nonetheless, he will see reflected in equipment 
design and philosophy the practical problems of co- 
ordinating hundreds of minds working under differ- 
ent conditions. What may seem to be a technical 
fault may, in fact, be attributable to the complex- 
ities of human endeavor. And again, a particularly 
elegant handling of a knotty situation may have been 
synthesized from the ideas of many minds. 

Radar was the fruit of several thousand scientists 
and engineers both in and out of military organiza- 
tions. The chapters to follow cannot adequately ex- 
press the importance of each contributor to the 
whole. We have learned much about cooperation and 
the difficulties of achieving it. We are grateful to all 
those agencies, military and industrial, who made our 
job easier by their generally friendly spirit of assist- 
ance. What we have said in criticism of civilian-mili- 
tary cooperation has been said in the hope of improv- 
ing future relations. Clearly these relations have been 
essentially healthy, even if sometimes erratic, be- 
cause this team did produce and employ in combat 
vast quantities of good equipment. It is only because 
war is so terribly inefficient that much room is left for 
improvement and criticism. 


RESTRICTED 



PART I 

RADAR MEANS FOR DETECTION OF SURFACE VESSELS 
FROM AIRCRAFT 


RKSTKK 'Tl'.l ) 



/ 


Chapter 2 

FUNCTIONS OF ASV RADARS 


2.1 GENERAL DESCRIPTION 

Aircraft to surface vessel [ASV] radar is an airborne 
radar whose primary function is to detect objects of 
interest (commonly called targets) on the surface of a 
body of water and to present adequate range and 
bearing data so that an observer in the aircraft may 
guide the aircraft close enough to a detected object 
for visual contact. All the ASV radars discussed in 
Part I are pulsed radars. 

2.1.1 Target Detection 

A beam of very high-frequency radio waves is di- 
rected from the aircraft and covers an area on the 
surface of the water which is large compared with the 
objects to be detected. In general, this is done by use 
of a parabolic reflector, with its associated antenna, 
which rotates through 360 degrees. Very little radio- 
frequency energy is reflected back from the surface 
toward the radar’s receiving antenna if the surface of 
the water is relatively flat. Moreover, the objects of 
interest are usually good reflectors presenting a fa- 
vorable aspect for reflecting a readily detectable 
amount of energy back to the radar. Thus, the pri- 
mary function of ASV radar is attained rather easily 
under good conditions. Under adverse conditions, 
however, such as rough air and rough sea, and when 
the object of interest happens to be small, this func- 
tion becomes extremely difficult to attain. When the 
sea is rough, the waves themselves reflect the radio 
waves back to the radar, causing a background of 
fluctuating signals. This background, commonly 
called sea clutter or sea return, tends to obscure the 
desired signals, and small targets may be completely 
lost. Sea clutter, then, often becomes a limiting factor 
in the usefulness of an ASV radar system. Conse- 
quently a criterion of good design in ASV radars is 
the extent to which sea clutter is eliminated or mini- 
mized. 

2.1.2 Navigational Feature 

The targets (objects of interest) which ASV radars 
are designed to detect may be shipping, buoys, is- 


lands, reefs; in fact, they may be practically anything 
on the surface of the water or the shoreline itself. It 
is implied, therefore, that an ASV radar serves in part 
as a navigational aid. Indeed, the more modern ASV 
radars, with plan position indicator [PPI], and in- 
creased resolution, have tended to emphasize the 
navigational feature as a major function. Since shore- 
lines, as a rule, give excellent radar signals, the map- 
like type of presentation afforded by ASV sets 
equipped with PPI makes such sets invaluable navi- 
gational aids. The use of radar for bombing, as de- 
scribed in Part II of this book, was largely an out- 
growth of the use of ASV radars for navigation. 

2.2 USES OF ASV IN PEACETIME 

2.2.1 Types of Missions ASV 

Radars May Perform 

Some of the peacetime uses of ASV radar are fairly 
obvious. Rendezvous at sea between ship and plane 
is easily accomplished when the plane is equipped 
with such a radar. The value of such a rendezvous is 
made evident by an incident in the North Atlantic. 
A critically ill merchant seaman was removed from a 
freighter to a Coast Guard PBM flying boat and 
flown safely to land for medical attention. The 
PBM’s ASV radar had guided the plane to the ship 
through a heavy fog. Rescues at sea and deliveries of 
medical supplies and personnel to ships and small 
islands are other life-saving missions for which ASV 
radars are especially adapted. A less spectacular and 
more commonplace type of mission for the peacetime 
ASV radars is the rendezvous at sea for the purpose 
of transferring mail. 

2.2.2 Beacons 

Radar beacons 6 have proved a valuable adjunct 
to ASV radars during the war, primarily as naviga- 
tional aids, and doubtless will continue to do so. The 
radar beacon is designed to respond by sending out a 
pulse or sequence of pulses whenever it receives a 
pulse from a radar whose transmitter frequency falls 
within a certain frequency band. Beacons have the 


I 


RESTRICTED 


r 


9 


10 


FUNCTIONS OF ASY RADARS 


additional feature that they respond only to trans- 
mitted pulses whose pulse durations are within cer- 
tain limits. This feature enables the operator of the 
radar to exercise control over the beacon, so that it 
will respond to his radar only if the operator so de- 
sires. The response of the beacon is usually coded for 
identification purposes. Also the beacon’s transmitter 
frequency is out of the frequency band of the radar’s 
transmitter to prevent radar signals from obscuring 
the beacon response. Some of the specialized require- 
ments of such a radar beacon are immediately evi- 
dent. For example, its receiver bandwidth must be 
fairly large and its antenna pattern broad. Corre- 
spondingly, design requirements are imposed on the 
ASV radar to insure appropriate interrogation of and 
reception from the beacon. Beacons are located usu- 
ally at favorable shore locations or on ships. 

Some of the uses of ASV radars mentioned above 
would be enhanced by the use of beacons. The mail 
transfer operation, in particular, is one which would 
be especially benefited by the use of a radar beacon 
on the ship, since it would avoid confusion if other 
ships happened to be near the rendezvous area and 
since it would eliminate the sea clutter problem. 

2.3 MILITARY APPLICATIONS 

2.3.1 Routine Patrol 

The chief military function of ASV radar during 
the war was for patrol, usually routine in nature, to 
insure that no enemy ships entered the area under 
patrol. It was largely through performance in mis- 
sions of this type that the ASV radars earned the ap- 
preciative soubriquet “the eyes of the fleet.” Not 
only were the radars useful during time of poor visi- 
bility, but even on the clearest days they could be 
relied upon to detect ships of all sizes at greater 
ranges than those at which they could be detected 
visually. For these reasons, fair weather or foul, the 
ASV patrol or search missions were flown on routine 
schedules in so far as possible. As a rule, medium and 
heavy bombers were used in this work, though small 
bombers and torpedo bombers were also used to ad- 
vantage. In order that the plane be able to patrol as 
large an area as possible, it is usually desirable that 
the two paramount design features of ASV radars be 
maximum range performance and minimum weight. 
Since these two features impose rather contradictory 
requirements, the design of an ASV radar set always 
involves a certain amount of compromise between 
them. 


An ASV radar search mission will be described 
briefly, beginning with the briefing of the pilot. For 
the purposes of this discussion, briefing for such a 
mission includes information regarding friendly ves- 
sels in the area and instructions regarding such things 
as weather conditions and radio watches. The radar 
operator, who may also be the navigator or radio 
operator, may be briefed separately or with the pilot, 
or he may simply receive such information from the 
pilot. During the mission itself the radar operator re- 
ports to the pilot upon noting a signal. (Some ASV 
radars provide a second scope for the pilot, though 
this is not usually the case.) If there is any suspicion 
that the signal is from an enemy vessel the pilot in- 
forms his operations base. Following this initial con- 
tact any of a great variety of maneuvers may be 
made subject to weather conditions, time of day, and 
the prevailing tactical situation. 

Frequently the plane is ordered to “home” on the 
suspicious vessel for the purpose of identifying it and 
possibly to attack it. This homing operation consists 
of flying a course as directed by the radar operator 
to intercept the target; it is an important feature in 
ASV radars. 

2.3.2 Submarine Detection 

One of the most important roles played by ASV 
radars during the war was in antisubmarine warfare. 
Probably the earliest successful operational use of 
American ASV radars against the enemy was in pa- 
trolling the Eastern Seaboard area of the United 
States and the Caribbean area during the most of 
1942, when the German submarines constituted a 
very serious threat to Allied shipping in the Atlantic. 
The tactical situation existing during these opera- 
tions was such that an attack was initiated as soon as 
it was established that a suspicious signal came from 
an enemy submarine. This attack was usually carried 
out with depth bombs. Although confirmed sinkings 
of the submarines were few compared with the num- 
ber of attacks made by the planes themselves, the 
effect upon the morale of the submarine crews, to- 
gether with the sinkings made by destroyers and sub- 
chasers called to the scene by these planes, soon 
caused the submarines to withdraw to happier hunt- 
ing grounds, at least temporarily. A race of armament 
and tactics between the German submarine experts 
and American and British antisubmarine experts 
soon was under way. 

The one most obvious tactic for a submarine to 


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BIB 



MILITARY APPLICATIONS 


11 


employ, when a hostile plane is approaching, is to 
dive beneath the surface where the submarine is safe 
from detection by eye and by radar. One of the first 
important countermeasures was the use of a mag- 
netic device for locating the submerged submarines. 
This device, known as magnetic airborne detector 
[MAD], was a sort of airborne magnetometer. Its 
range, only a few hundred feet, was short compared 
with the radar’s range, but the fact that it could de- 
tect submerged submarines made it an important ad- 
junct to ASV radar, by enabling the plane to drop 
depth charges on the submarines and to drop pat- 
terns of marker buoys to aid friendly vessels called to 
the scene. Later in the war, sono buoys were used to 
replace the MAD and marker buoys. 

One of the earliest antiradar devices used by the 
submarines was a receiver for detecting radar signals 
from the aircraft. These receivers were sufficiently 
sensitive to detect such signals at ranges comparable 
to, and often exceeding, the ranges at which the air- 
craft could detect the surfaced submarines. The chief 
countermeasure against these receivers was a shift to 
radars of much higher frequencies, the microwave 
radars, and having these radars operate over a suffi- 
ciently large frequency band to decrease considerably 
the probability of the receivers detecting them. A 
second measure designed to thwart the Germans’ 
microwave receivers was the Vixen attachment for 
ASV radars. 

Normally if a submarine is listening to a plane’s 
radar and if the plane is approaching as in a homing 
run, the power of the received signal varies inversely 
as the square of the distance between submarine and 
plane. Thus, the variation in the power of the re- 
ceived signal gives the submarine its cue as to 
whether the plane is homing on the submarine. The 
Vixen attachment is a cam-operated variable attenu- 
ator driven by a motor so as to make the transmitted 
power vary as a function of range, in such a manner 
that the power received at the submarine remains 
constant or decreases as the plane approaches. The 
operation of Vixen during a homing run is explained 
by the following equations: 

kP , k'P 

Py = - and p. = — 

where P is the transmitted power, p p the power re- 
ceived at the plane due to reflection from the sub- 
marine, p 8 the power received by the submarine re- 
ceiver, R the distance between plane and submarine, 
and k and k' are positive constants depending upon 


receiver and antenna characteristics. Now suppose 
the Vixen cam is constructed so that it varies P in 
such a manner that the relation P = aR z is valid, 
where a is a positive constant. By substituting in the 
above equations, 

p P = °~ and p 8 = ak'R. 

R 

Thus, the power received at the submarine decreases 
as the range decreases (presumably causing the lis- 
tener on the submarine to believe the plane is going 
away) , whereas the signal received at the aircraft in- 
creases as the range decreases, though not so rapidly 
as when the Vixen is not used. 

Actually, the Vixen device received little use, 
largely because other measures, such as the frequency 
change mentioned above, together with the destruc- 
tion of enemy bases, factories, and other equipment 
on the European continent, succeeded in minimizing 
the submarine threat. 

The last important device used by the German 
submarines in defense against ASV radars posed seri- 
ous problems for the Allies. This device, known as 
Schnorkel , 7 was a revolutionary one in submarine op- 
erations, in that it enabled the submarine to stay 
submerged almost indefinitely and also to travel un- 
der water at much faster speeds than previously. The 
portion of the Schnorkel above the water line is suffi- 
ciently small to make it a poor radar target, espe- 
cially when covered with radar absorbent material or 
in the presence of much sea clutter. Chief features of 
ASV radar design to counteract the Schnorkel were 
narrow-beam antennas and shorter pulse duration 
(to increase the signal-to-sea-return ratio), and spe- 
cial receiver circuits for minimizing the effects of sea 
clutter. 

The use of escort carriers with convoys in the At- 
lantic widely extended the area under surveillance of 
ASV radars and added greatly to the demise of the 
submarine menace. 

2.3.3 Other Uses 

Another important role of ASV radar during the 
war was its use by blimps for aiding in convoy escort 
duty. It was used to aid the blimps in making contact 
with their convoys and in patrolling the area around 
the convoys for submarine detection. 

ASV radars were employed successfully on numer- 
ous occasions for rescue at sea of survivors of sunken 
vessels and of planes forced down at sea. A simple, 
but extremely useful contrivance, adopted for assist- 


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12 


FUNCTIONS OF ASY RADARS 


ing the radar in such missions is the collapsible comer 
reflector for use on life rafts. A corner reflector con- 
sists of three microwave reflectors (for example, 
fabric knitted from cotton thread which is wrapped 
with a silver-plated spiral) placed at right angles on 
an aluminum framework forming the corners of a box. 
Since life rafts make rather poor radar targets and 
corner reflectors make good ones, if correctly ori- 
ented, these reflectors increase greatly the range at 
which the rafts can be detected by radar. 

It was mentioned above that ASV radar served an 
important function in guiding the plane to its target. 


In fact, such operations are usually successful in 
guiding the plane almost directly over a ship. In gen- 
eral, however, such homing runs carried out with the 
aid of ASV radar alone are not of the precision requi- 
site for successful bombing, though the radar is 
doubtless sufficiently accurate for facilitating some of 
the functions mentioned above, such as the dropping 
of supplies. The homing run may well terminate in a 
bombing run, in which the actual dropping is done 
with the aid of an optical bombsight or a radar bomb- 
ing attachment of one of the types to be described in 
Part II. 


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ED 


Chapter 3 

MEANS OF ASV DETECTION 


Two types of aircraft to surface vessel [ASV] radars 
widely used by the United States were those operat- 
ing at transmitter frequencies below 600 me (long 
wave) and those operating at transmitter frequencies 
above 3,000 me (microwave); the former were the 
earlier type, the latter the more recently developed. 
There were no ASV radars of importance with trans- 
mitter frequencies between these limits. 

3.1 SEMIDIRECTIONAL EQUIPMENT 

A characteristic difference in external appearance 
of the microwave ASV radars and those of lower fre- 
quencies is in the type of antennas used. Microwave 
ASV radars use parabolic or modified parabolic re- 
flectors for directing beams of high-frequency elec- 
tromagnetic waves, and scanning systems to enable 
the beams to cover large areas of the surface and to 
obtain bearing data on reflecting objects. The lower- 
frequency radars employ fixed directional antenna 
arrays of the Yagi type, Ua for directing the waves and 
obtaining bearing information on reflecting objects; 
the amplitudes of the signals received from the ob- 
jects on two fixed antennas are compared. A typical 
Yagi array for ASV use consists of one driven an- 
tenna, one parasitic antenna, and two parasitic direc- 
tor antennas. The major portion of the present chap- 
ter is devoted to microwave ASV radars. First, how- 
ever, two of the lower frequency radars which were 
used rather widely and successfully are described. 

3.1.1 Search and Homing Antenna 

for AS VC Radar 

The first ASV radar to be used extensively by the 
United States forces was of British and Canadian de- 
sign, and operated on a nominal frequency of 176 me. 
For general search use it employed a single wire 
transmitting antenna and two receiving antenna ar- 
rays aligned with main lobes extending directly to 
port and to starboard of the aircraft (Figure 1A). For 
homing, it employed a Yagi transmitting antenna ar- 
ray, located near the nose of the aircraft and aligned 
so that its main lobe was directly along the axis of the 
aircraft, and its receiving antennas located under 
each wing of the aircraft aligned so as to have over- 
lapping lobes, as illustrated in Figure IB. This radar 
was called AS VC. An American redesign of this 
equipment for the Navy was known as ASE. 


DIRECTION OF FLIGHT 



ANTENNA LOBES 
A "SEARCH" 


DIRECTION OF FLIGHT 




C INDICATOR PRESENTATION 

Figure 1A, B, C. Low-frequency ASV antenna pat- 
terns and indicator. 

3.1.2 Data Presentation in Longer- 
Wave ASV Radars 

The ASV indicator, typical of the longer wave ASV 
radars, employs an electrostatic cathode-ray tube. A 


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. 


14 


MEANS OF ASV DETECTION 


linear saw-tooth sweep, synchronized to cause the 
electron beam to start upward each time the trans- 
mitter fires, is applied to the vertical plates of the 
tube. Signal voltages from the two receiving anten- 
nas are applied to the horizontal plates of the tube in 
such a manner that signals from the port antenna 
cause deflections of the electron beam to the left, and 
signals from the starboard antenna cause deflections 
of the electron beam to the right. A motor-driven 
switching device connects the receiver first to one re- 
ceiving antenna and then to the other for equal inter- 
vals of time. 

As illustrated in Figures IB and 1C, the port an- 
tenna receives more reflected energy from the reflect- 
ing object shown than does the starboard antenna. 
Hence, the electron beam of the indicator tube is de- 
flected more to the left than to the right. The relative 
sizes of the two deflections are an indication of the 
bearing of the object. The vertical position on the 
tube at which the deflections occur is a measure of 
elapsed time since the firing of the transmitter and is 
therefore a measure of range. By changing the slope 
of the saw-tooth sweep voltage, various range scales 
may be selected. The radar operator guides the plane 
in a homing run by giving the pilot directions to keep 
on a course such that the deflections to the left and to 
the right are of the same amplitude. The technique of 
switching from one antenna to another as a means of 
accurate direction finding is known as lobe-switching. 
Most radars used for obtaining accurate bearing and 
elevation data use a refinement of this same tech- 
nique. 


frequency is 515 me. The pulse duration is approxi- 
mately 2 jusec and its pulse recurrence frequency ap- 
proximately 400 pulses per second. The motor-driven 
antenna switching unit switches from one antenna to 
the other at a rate of thirty times a second. A novel 
hydraulic control system enables the use of the same 
antennas for search and for homing. In normal search 
operation the antennas are aligned so that their 
beams point at right angles to the direction of flight. 
In homing, the antennas are aligned so that their 
beams overlap, as in Figure IB. By the use of the hy- 
draulic control system the operator causes the an- 



Figure 2. Block diagram of major units of ASB radar. 


3.1.3 Description of ASB Radar 
System 

A significant improvement in operation of ASV 
radars of the type described above was brought about 
by the development of the duplexer, 3 an electronic 
switching device which enabled the use of a single an- 
tenna for both transmitting and receiving. The use of 
this device makes the nose array no longer necessary. 
First one wing antenna is used for both transmitting 
and receiving and then the other. This arrangement 
permits a more efficient use of the transmitted power. 
The U.S. Navy’s ASB radar, which employed this 
principle, was one of the most successful of all ASV 
radars. Figure 2 shows a block diagram of the major 
units of the ASB radar. 

The indicator used with the ASB radar is similar to 
the one described above. The nominal transmitting 


tennas to move from the search position to the hom- 
ing position. By continually using the control and by 
properly maneuvering the airplane, a good pilot and 
radar operator team can make the antenna beams 
scan a wide area quite effectively. Although lacking a 
number of desirable features of the microwave radars, 
the ASB’s reliability, versatility, and light weight 
made it one of the most useful of all ASV radars. 

3.1.4 General Characteristics of 
Long- Wave Radars 

Radar beacons for use with these longer- wave ra- 
dars are coded by means of motor driven coding 
wheels which key the beacon transmitter in a peri- 
odic sequence at a rate of about 1 cycle per second. 
Each radar pulse received by the beacon causes the 
beacon to send out a responding pulse unless the cod- 


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MILITARY APPLICATIONS 


15 


ing wheel shuts it off. The radar operator identifies 
the beacon by the fluttering sequence of the signal. 

It will be noted that none of these longer-wave 
ASV systems gives quantitative bearing data, al- 
though they are adequate for the homing function. 
For this reason, these radars are far inferior to the 
microwave radars as navigational aids. 

Under good operating conditions, ASV radar sys- 
tems of the type described above give excellent per- 
formance in so far as detection of large objects at long 
ranges is concerned. The ASB radar described above 
is quite suitable in this respect, although its average 
output power is only about 12 watts. The usefulness 
of these radars, however, is limited by their lack of 
resolving power. The degree of resolution of a radar 
determines its ability to separate the signals received 
from objects close together, both in range and in 
azimuth. In an ASV radar, it also determines its abil- 
ity to discriminate between a desired signal and un- 
desired sea clutter. Resolving power is primarily a 
function of the area of the surface illuminated by a 
single pulse. It is inversely proportional to the an- 
tenna beamwidth and inversely proportional to the 
pulse duration. It is virtually impossible to secure a 
narrow beam comparable to that used in the micro- 
wave radars (less than 10 degrees between the half- 
power points) at the frequencies employed by the 
longer-wave radars, with an antenna capable of being 
airborne. Although later models of the ASB radars 
incorporated a number of anticlutter 4a circuits de- 
veloped for minimizing sea return, the performance of 
these radars in the detection of small objects in a 
rough sea left much to be desired. Against the 
Schnorkel, for example, the ASB radar is almost 
useless. 

3.2 MICROWAVE SCANNING SYSTEMS 

Scanning systems used with the microwave ASV 
radars give these radars an important feature not ob- 
tained with the longer-wave ASV radars, namely, 
quantitative bearing information. The high-gain 
parabolic reflectors used for directing the microwave 
beams result in beams sufficiently narrow that com- 
paratively accurate bearing data are readily ob- 
tained. Microwave ASV radars generally use me- 
chanical scanning systems for moving the beams in 
such a way as to illuminate efficiently a large area of 
the surface in a periodic fashion (either a 360-degree 
scan or scan of a sector, 60 degrees for example, in 
front of the aircraft). 


3.2.1 Presentation of Data 

The microwave ASV radars generally employ one 
of two types of data presentation developed for mak- 
ing adequate use of the accurate bearing information 
obtained. One type, the plan position indicator [PPI], 
is a polar coordinate system in which the position of 
the aircraft is the pole. Signals received from objects 
produce bright spots on the tube. The distance from 
the pole to a spot is proportional to the slant range to 
the reflecting object, and the angular position corre- 
sponds to the bearing of the object. The other type of 
presentation, known simply as B scope presentation, 
is a rectangular coordinate system in which bearing 
and range are abscissa and ordinate respectively. 
This type of presentation is used chiefly by radars de- 
signed for installations where it is convenient to scan 
only the area in front of the aircraft. In both types, 
the received signals are applied to a cathode-ray tube 
in such a manner as to modulate the intensity of the 
electron beam. The cathode-ray tube screens used 
with both types of presentation have fairly long per- 
sistence so that the bright spots do not fade out com- 
pletely between successive scans. 

Navigational Feature 

The PPI display is a fairly accurate map, the main 
distortion being due to the fact that slant range is 
used rather than range measured along the surface. 
The distortion is negligible for most ASV work since 
the planes usually fly at low altitudes. The sweep ap- 
plied to the cathode-ray tube is modified in some sys- 
tems in which extremely high fidelity in mapping is 
desired, so as to approximate ground range data. The 
B scope display is a distorted map, the distortion be- 
ing greatest at short range. Although it is objection- 
able in some respects, the increased angular resolu- 
tion at close ranges afforded by this distortion is often 
found desirable in the final stages of a bombing run. 
Both types of presentation bear sufficient resem- 
blance to maps that coast lines, islands, lakes, rivers, 
and other geographical features may be readily iden- 
tified. The ASV radars are, therefore, most valuable 
navigational aids. (For a more detailed discussion of 
radar mapping systems and of the types of indication 
employed, see Section 7.2.) 

Other Features 

In addition to the geographical data mentioned 
above, other types of information obtained by these 
radars are often of value. Among these are signals 


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16 


MEANS OF ASV DETECTION 


from storms, from nearby airplanes, and the altitude 
signal. The altitude signal usually appears as a ring 
on the PPI and as a horizontal line on a B scope, and 
is due to the energy scattered more or less at random 
from the radar antenna and reflected back from the 
nearest point on the surface beneath the aircraft. 

Beacons for use with microwave ASV radars oper- 
ate similarly to the beacons used with the longer- 
wave ASV radars, with the exception that the coding 
technique is different. This difference is due to the 
different types of presentation used in the two types 
of radars. The microwave beacon employs a pulse 
duration discriminator, so that it responds only to 
pulses whose durations are approximately nsec. 
By selecting the pulse duration the radar operator 
can control the response of the beacon. The controls 
of the ASV radar customarily are such that when the 
operator wishes to interrogate the beacon, he oper- 
ates a control which simultaneously selects the cor- 
rect pulse duration and also tunes the local oscillator 
in the radar receiver to the correct frequency for re- 
ceiving the beacon signal. When the beacon is interro- 
gated, each radar pulse causes the beacon to respond 
with a definite sequence of pulses. The number of 
pulses of the sequence and the time interval between 
pulses of the sequence represents a code. The time 
interval between the firing of the radar pulse and the 
reception of the first pulse of the beacon sequence 
represents the range of the beacon when displayed on 
the radar’s indicator in the same manner as ordinary 
signals. Similarly, the bearing of the signals received 
from the beacon indicate the bearing of the beacon, 
since the beacon responds only when the radar an- 
tenna is pointed toward it (see Chapter 10) . At close 


ranges, the beacon may respond to side lobes of the 
antenna pattern, but the operator, by correct use of 
the receiver gain control, can usually distinguish be- 
tween the response to the main lobe and the responses 
to these side lobes. 

3.2.2 Comparison with Over-Land 
Scanning Systems 

Scanning systems employed by microwave ASV 
radars are essentially the same as those used by the 
radars developed primarily for bombing inland tar- 
gets. The antennas, however, are usually somewhat 
different in the two types of radars. The chief reason 
for this difference is that the radars designed for 
bombing inland targets must be capable of good map- 
ping of the ground from high altitudes, whereas ASV 
radars are generally used at comparatively low alti- 
tudes. In order that a radar give good illumination of 
the ground from high altitudes, the antenna beam 
must be fanned rather broadly in the vertical plane. 
This broadening of the beam is accomplished only at 
the sacrifice of antenna gain, and hence a reduction 
in the maximum range obtainable. In ASV radars, 
some fanning of the beam in the vertical plane is usu- 
ally desirable for good performance at close ranges, 
but since these radars are usually operated at lower 
altitudes, and since good performance at long range 
is a primary design consideration, the beam is usually 
fanned much less than in the radars designed prima- 
rily for bombing land targets. The AN/APS-15 radar, 
which was used extensively for both over-land bomb- 
ing and for ASV patrol work, provided different an- 
tennas for the two functions. 




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Chapter 4 

DESIGN CONSIDERATIONS 


4.1 BASIC DESIGN CONSIDERATIONS 

The design of a microwave aircraft to surface vessel 
[ASV] radar must necessarily represent a compro- 
mise between performance objectives and physical 
limitations. The objectives include good search per- 
formance at both long and short ranges and high 
range and azimuth resolution; the physical limita- 
tions are those of size, weight, and power consump- 
tion. The basic design considerations essential to 
effecting a satisfactory compromise between these 
objectives and limitations are given in the present sec- 
tion. In addition to the basic features discussed here, 
there are many other design considerations affecting 
the general utility of the radar. Among these are re- 
liability, simplicity of operational controls, facilities 
for test and maintenance, and facilities for accom- 
modating various useful attachments (such as bomb- 
ing and rocket-firing computers, and identification of 
friend or foe [IFF] equipment). 

4 . 1.1 Azimuth Resolution 

In Section 3.1.4 we indicated the importance of 
high resolving power in ASV radars, noting its desir- 
ability for separating signals received from objects 
close together and also for discriminating between a 
desired signal and undesired sea return. The azimuth 
resolution of a radar increases as the antenna beam- 
width in the horizontal plane decreases. The beam- 
width is the angle between the two half-power points 
of the antenna pattern. 

In a microwave ASV radar antenna the feed is 
placed approximately at the focal point of a parabolic 
reflector. High-frequency electromagnetic waves are 
radiated from the feed to the reflector, which concen- 
trates the waves into a beam of energy. The beam- 
width of such an antenna is directly proportional to 
the wavelength X and inversely proportional to the 
width of the reflector. Hence, maximum azimuth 
resolution is obtained by using the widest possible 
antenna and the shortest possible wavelength. How- 
ever, antenna size and weight are limited by the air- 
craft in which the radar is to be used and the type of 
installation contemplated. The limitations on wave- 
length are not so readily apparent. In the early stages 
of development of microwave radar, the efficiency of 
many r-f components decreased rather rapidly as the 


wavelength decreased. However, at the close of 
World War II, the long-range performance of most 
X band ASV systems was about as good as with 
S band systems of about the same weight and power 
consumption, since both were capable of detecting 
medium sized vessels at distances comparable to the 
horizon distance at the altitudes favored for ASV 
operations. K band systems at that time were defi- 
nitely inferior in long-range search performance be- 
cause of (1) the limited output power of K band 
transmitting tubes, (2) the relative inefficiency of 
some of the r-f components, and (3) the attenuation 
of K band waves by moisture in the atmosphere. 

4.1.2 Pulse Duration 

Two of the objectives in ASV radar design, good 
long-range search performance and good range reso- 
lution, are largely incompatible with each other in so 
far as pulse duration is concerned. The degree of 
resolution in range is fundamentally limited by the 
pulse duration — the shorter the pulse the greater 
the range resolution. Long-range search performance, 
on the other hand, varies with pulse duration in an 
opposite manner — the greater the pulse duration, 
in general, the greater the range at which a given ob- 
ject may be detected. More explicitly, the minimum 
discernible signal detectable by the receiver varies 
inversely with the product of the pulse duration and 
the square root of the recurrence frequency, provided 
the receiver bandwidth is kept at the optimum value 
for the pulse duration. In general, maximum range 
at which a discrete object may be detected varies in- 
versely with the fourth power of the minimum dis- 
cernible signal, provided the peak power is kept con- 
stant. 411 It is usually more expedient to use different 
pulse durations rather than to select a pulse duration 
both for long-range search and for resolution. The 
pulse recurrence frequency and maximum sweep 
range are usually varied simultaneously with the 
pulse duration. Average output power may then be 
kept fairly constant. If its average output power is 
kept fairly constant, the radar modulator is, in gen- 
eral, considerably simplified, and the transmitting 
frequency is usually more stable. Also, each change 
in pulse duration alone adds complexity to the modu- 
lator as well as to the control circuits. Furthermore, 
if the receiver bandwidth is adjusted to its optimum 


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17 


DESIGN CONSIDERATIONS 


18 ' 

-r. 1 - 

value for each pulse duration used, the receiver be- 
comes very complex indeed. In practice considerable 
compromise must be made in receiver band widths. 
At present, no airborne ASV receiver uses more than 
two bandwidths. 

Additional factors influencing the choice of pulse 
deviations, recurrence frequencies, and sweep lengths 
are now considered. 

The maximum range R m *x at which useful informa 
tion is obtained is fundamentally limited by the pulse 
recurrence frequency v r according to the formula, 

D _ A 

■ftmax n ) 

2v r 

where c is the velocity of propagation of the elec- 
tromagnetic waves (approximately 186,000 statute 
miles per second). This insures that long-range sig- 
nals do not appear on second or even on later sweeps. 
Actually, R max is considerably less than the value 
given by this formula, since the formula makes no 
allowance for the time required for the sweep on the 
cathode-ray tube to return to its initial position, nor 
does it allow for tolerances in circuit components. 

Regardless of the degree of resolution of which a 
radar is capable, we are making full use of its resolv- 
ing power only if the data are presented on the tube 
in the form of a map of sufficiently large scale — in 
other words, only if a sufficiently short range sweep 
is used. If a 100-mile sweep is used in normal plan 
position indicator [PPI] presentation, and if a stand- 
ard 5-in. cathode-ray tube is used as the indicator, 
then the map obtained has a scale of 40 miles to the 
inch. High resolution is impossible in such a case. As 
a fairly typical example, suppose that a 100-mile 
sweep range for search is desired, maintaining an 
average power output of 40 watts. This may be ac- 
complished by using a 2-psec pulse duration and a 
recurrence frequency of 800 pulses per second. (Some 
PPI presentation schemes would require a consider- 
ably lower recurrence frequency because of a long 
sweep recovery time.) The peak power output is 
25 kw, obtained from the relation P avg X l/v r = Pt, 
where P avg is the average power, v r is the pulse recur- 
rence frequency, P is the peak power, and r is the 
pulse duration. 

Essentially the same average power output as well 
as peak power output can be maintained, and good 
range resolution obtained when a short range sweep 
is used, by using a recurrence frequency of 3,200 and 
a pulse duration of % //sec. This combination gives a 
resolution in range about the same as the azimuth 


resolution obtainable at a slant range of 5 miles with 
an antenna beamwidth of Yi degree. (It would re- 
quire an antenna 15 ft wide to obtain such a beam- 
width at X band.) Targets separated in range by 
about 250 ft can be differentiated. The maximum 
range obtainable with this combination is about 25 
miles, a fairly useful range for submarine search in 
modern ASV systems. For some uses, such as accu- 
rate bombing and detailed presentation of harbor in- 
stallations, a much shorter range sweep presentation 
is desired; but little is gained, in comparison to the 
cost in complexity of the system, by further increas- 
ing the recurrence frequency and decreasing the pulse 
duration. In the example just cited, the peak pulse 
power could have been kept the same, and the pulse 
duration decreased without increasing the recurrence 
frequency. In this case, only one-fourth the average 
power used in the long-range search operation is avail- 
able, so that some of the weaker signals from close 
objects probably would be missed. 

4.1.3 Stabilization 

Microwave ASV radar design often involves con- 
sideration of various techniques for stabilizing the 
antenna. Under ordinary conditions, none of these 
stabilization techniques is a necessity in enabling the 
radar to carry out the primary ASV functions of de- 
tection and homing. Moreover, most of these tech- 
niques add considerably to the weight and power 
consumption of the radar. The chief virtues of these 
techniques are that they make the operator’s job 
much easier and improve the performance of the 
radar under adverse conditions and for some specific 
applications. These virtues are of sufficient impor- 
tance to warrant description of some of the stabiliza- 
tion techniques which have been found useful in 
ASV radars. 

Line-of-Sight Stabilization 

The object of line-of-sight stabilization is to keep 
the antenna beam directed to secure good illumina- 
tion of the surface of the ground or sea regardless of 
the changes in attitude of the aircraft. The use of 
line-of-sight stabilization helps prevent the loss of 
signals during ordinary maneuvers and during pitch- 
ing and rolling caused by rough air. This stabilization 
is accomplished by automatically adjusting the an- 
tenna tilt so that the angle between the true vertical 
and the point of maximum power of the antenna 
beam is held approximately constant, at the value 
selected by the operator’s manual tilt control, for all 


ItK 


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BASIC DESIGN CONSIDERATIONS 


19 


azimuths scanned and for all attitudes of the airplane 
attained during ordinary maneuvers. The automatic 
adjustment of the antenna tilt is performed by a 
servo system controlled by a vertical gyroscope. The 
servo must be capable of following fairly high rates 
and accelerations, and it works continuously when 
the aircraft is in any attitude except the normal 
flight attitude at which the servo is “zeroed. ” These 
requirements generally cause the servo to be rather 
heavy. 

If high accuracy of presentation is desired for all 
attitudes of the aircraft, an azimuth correction of the 
data must be employed, since the tilt axis is generally 
not horizontal. In some bombing applications, high 
accuracy of presentation is required only at azimuths 
near the heading of the aircraft. Such accuracy may 
be obtained electronically by shifting the data later- 
ally on the face of the indicator tube by the amount 
h tan 6, where h is the altitude and 6 is the angle of 
roll. For small roll angles the approximation hd or 
h sin 6 may suffice. A voltage proportional to d or 
sin d may be made available from a potentiometer or 
synchro on the roll axis of the gyroscope, and a fairly 
simple computer may be constructed for supplying a 
voltage proportional to hd or h sin 6. The lateral shift- 
ing of data may be accomplished by applying this 
voltage to a horizontal centering coil such as is fre- 
quently used with electromagnetic cathode-ray 
tubes. 

Platform Stabilization 

Platform stabilization serves to keep the base of 
the scanning system always on a horizontal plane re- 
gardless of the maneuvers of the aircraft. It is an 
ideal type of stabilization, since it obtains both good 
illumination of the surface of the ground or sea and 
also accurate azimuth data. It is necessary to provide 
a gimbal mount for the entire scanning system to 
keep the base of the scanner horizontal during rolling 
and pitching of the aircraft. Also, a great amount of 
space must be provided so that the scanner can per- 
form the necessary rotations. At the close of World 
War II, platform stabilization had been used only in 
experimental bombing systems. The physical limita- 
tions of size and weight generally ruled out this type 
of stabilization for ASV radar systems. Platform 
stabilization is accomplished by means of two servo 
systems which independently adjust the base of the 
scanner to compensate for the roll and pitch of the 
aircraft. These servos are controlled by synchros or 
potentiometers on the roll and pitch axes of a vertical 


gyroscope. The servos are in some respects simpler 
than the servo required for line-of-sight stabilization, 
since they have to work only during the times when 
the attitude of the aircraft is actually changing, 
whereas in line-of-sight stabilization the tilt is varied 
as a function of azimuth angle. 

The mechanical problems of platform stabilization 
are partially avoided by the use of roll stabilization , in 
which case the scanner base is adjusted in roll only. 
Such a stabilization system also partially avoids one 
of the difficulties of line-of-sight stabilization in that 
good azimuth accuracy in presentation is preserved 
for azimuths near the heading of the aircraft. How- 
ever, when the aircraft is in a climb or dive, good il- 
lumination of the ground or sea at azimuths near the 
heading of the aircraft is not automatically secured; 
nor is the azimuth accuracy of presentation main- 
tained at azimuths approximately at right angles to 
the heading. 

Compass Stabilization 

The chief object of compass stabilization is to pre- 
vent the smearing of signals on the indicator tube 
caused by changes in heading of the aircraft. If com- 
pass stabilization is not used, the map on the indi- 
cator tube rotates as the heading changes. This rota- 
tion of the map, together with the persistence of sig- 
nals on the indicator tube, results in a smearing of the 
tube face which may cause the loss of signals until 
the tube face clears. The use of compass stabilization 
generally makes the two types of gyroscope stabiliza- 
tion discussed above much more effective. Also it 
may assist, at times, in the solution of navigational 
problems. 

Compass stabilization is used only in systems em- 
ploying PPI presentation. In airborne radars it is 
usually employed in such a manner that the map on 
the indicator tube is always presented with north at 
its top. This stabilization is accomplished by means 
of a servo system controlled by a directional gyro- 
scope. The servo rotates the synchro normally used 
for transmitting azimuth data to the indicating tube 
in such a manner as to add the compass heading data 
to the normal bearing data. It is customary, when 
compass stabilization is employed, to cause a bright 
line to appear on the indicator tube at the compass 
bearing corresponding to the heading of the aircraft. 

Range Stabilization 

The purpose of range stabilization is to keep the 
signal from a particular object of interest always at 


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20 


DESIGN CONSIDERATIONS 


the same position on the face of the indicator tube. 
This can be accomplished by an automatic continu- 
ous variation of sweep length during a homing run on 
the object. Aside from a specified application to low 
altitude bombing [LAB] (see Section 8.4), this type of 
stabilization has received little use up to the present 
time, but probably would be particularly useful in 
following a weak intermittent type of signal, such as 
that from Schnorkel in the presence of sea return. 
Once the desired signal has been detected and the 
range stabilization put into operation, an apparent 
gain in signal-to-noise and signal-to-sea-return ratios 
is effected upon the indicator tube, since the noise 
and sea-return signals occur at random, whereas the 
desired signal is built up on a screen of long persist- 
ence by always appearing at the same spot on the 
tube, even though the signal may be missed com- 
pletely on some scans of the antenna. 

4.1.4 Sea Return 

The importance of high resolution in minimizing 
the effects of sea return was mentioned in Section 
3.1.4. It was noted also that the resolution is in- 
versely proportional to the pulse duration. Thus, in 
order to make the performance of the radar as ef- 
fective as possible in the presence of sea return, the 
design of the radar should incorporate the narrowest 
beam and shortest pulse duration consistent with the 
attainment of the other objectives of the design, and 
consistent with the physical limitations of size, 
weight, and power consumption. 

The polarization of the beam of electromagnetic 
waves directed from the radar antenna is also an im- 
portant consideration in the sea-return problem. Ex- 
periments indicate fairly conclusively that sea return 
is usually worse when the electric vector is vertical 
than when it is horizontal. Hence, if consistent with 
practical problems of antenna and scanner design, 
horizontal polarization is preferable for ASV radars. 

The antenna beamwidth, the pulse duration, the 
polarization, and possibly the wavelength determine 
the ratio of signal strength to sea return (except for 
effects of conditions external to the radar itself). 
There are, however, several electronic devices which 
may be incorporated in the radar receiver and indi- 
cator circuits to improve the presentation of the de- 
sired signal in the presence of sea return. These cir- 
cuits automatically operate upon the receiver and 
video circuits so as to reject the sea return as much as 
possible, without too great a loss in the desired signal, 


before the data are presented upon the indicator 
tube. 4a 

4.1.5 Scan Rate 

Another factor affecting the range of any radar 
system is the scanning rate. Maximum range is ob- 
tained if the radar beam is continuously focused on 
the target; but one of the functions of an ASV system 
is to search continuously over a large area, and it is 
only after a target has thus been found that the beam 
is pointed in one direction. Thus, it is necessary to 
consider carefully the scan rate to be used and the 
various scanner controls that are to be built into an 
ASV set. 

The so-called scanning loss S d b, in decibels, is de- 
fined as 

R 

S dh = 40 log — ° 

its 

where Ro is the “no scan” maximum range for a cer- 
tain target and R s is the maximum range at which this 
target can be seen while scanning. S d b varies with 
scan rate, type of scan, beamwidth, and other factors. 
A typical plot of scanning loss against scan rate 
shows that S dh is 2 db or less for scan rates of 5 rpm 
or less and rises to 12 db for scan rates over 30 rpm. 

The saturation scanning loss may be computed 
approximately from: 

Sdb = 5 l0g nTi 

where n is the fraction of time during which the beam 
illuminates the target, taken over a long period, and 
T i is an integration interval (in seconds) determined 
by the indicator screen persistence and the memory 
characteristics of the observer. It varies also with the 
amount of signal overlap that occurs on the screen, 
which depends on the aircraft speed, among other 
things. Ti is said to have a lower limit of about 8 sec. 

For the type of scan usually used with microwave 
ASV systems, namely, a 360-degree scan at uniform 
angular speed, n is the ratio of the azimuth beam- 
width to 360 degrees. For sector scanning through an 
angle 6 at a uniform rate, n is the ratio of the azimuth 
beamwidth to 6. We now discuss several implications 
of the foregoing points. 

For long-range ASV search, the range wdll be in- 
creased by a low scan rate, but the rate used must be 
compatible with the aircraft speed and the sweep 
length on the indicator tube. That is, the target must 



ENGINEERING CONSIDERATIONS 


21 


not travel an excessive amount on the screen between 
successive scans. For example, in a searching aircraft 
traveling at 300 knots and scanning at 5 looks per 
minute (5 rpm), the aircraft travels 1 nautical mile 
between successive looks. If an indicator sweep speed 
of 10 nautical miles per inch is used, the signals will 
move across the screen in discrete jumps of 2.5 mm. 
This is somewhat too large to take advantage of sig- 
nal build-up on a high persistence screen, and a some- 
what higher scan rate would seem advisable. For 
higher sweep speeds, a higher scan rate becomes defi- 
nitely necessary, and sector scan is valuable in hom- 
ing on a target because it gives many more looks per 
minute, thus allowing rapid course corrections. 

When designing an ASV system with the difficult 
and important problem in mind of searching for such 
small objects as the Schnorkel, a satisfactory com- 
promise must be effected between the minimization 
of sea return by narrowing the beam and the resulting 
scanning loss at the fairly high scan rates necessary. A 
more detailed treatment of scanning losses has been 
given by E. M. Purcell. 4b 

4.2 ENGINEERING CONSIDERATIONS 

The principal function of an ASV set is to detect 
and home on all types of surface vessels at the maxi- 
mum possible range, as pointed out previously. At 
present this principal function divides naturally into 
two parts; (1) long-range detection of sizable vessels 
and (2) necessarily short-range detection of small 
objects, such as the Schnorkel. A secondary, but still 
important function of the ASV set is to provide navi- 
gational information. 

The above functions of the ASV system affect the 
basic design of the set, making it more complicated 
in circuits and controls than a simple radar mapping 
set. In the same way, the special ASV functions affect 
the engineering considerations. General airborne 
radar engineering problems are considered in the 
R. L. Technical Series, 5a and in Section 5.1 below 
engineering problems as related to maintenance are 
discussed. This section will accordingly treat only 
those engineering problems that arise from the special 
functions of the ASV systems. 

4.2.1 Set Design 

In any piece of equipment designed to be airborne, 
weight is a vital factor, but under certain conditions, 
the word “vital” has to be qualified. In military 


bombing aircraft, the value of a mission is not meas- 
ured by the total load carried, but rather in the total 
damage done to the enemy. Thus, in a bombing air- 
craft, bomb load may have to be sacrificed for an ac- 
curate bombsight, and in an ASV plane, bomb load 
or range may have to be sacrificed for improved abil- 
ity to find enemy vessels. Since improvement in radar 
range means that the aircraft does not have to travel 
so far to search a given area, and since in general, in- 
creased weight in a radar set gives increased radar 
range, it may happen that a heavier radar set will 
allow the patrol area of a search aircraft to be in- 
creased, or alternatively will allow the patrol area to 
be kept the same and the bomb load increased. 

Another factor that is vital in all airborne equip- 
ment is reliability. In the very long search missions 
undertaken by ASV planes, reliability becomes even 
more important, since the effectiveness of the whole 
mission depends on the continued operation of the 
radar set. Thus the ASV system must be designed for 
reliable operation over long periods of time, and even 
an increase in weight should be tolerated in order to 
obtain this reliability. A corollary to reliability in an 
ASV set is easy accessibility to fuses, tuning controls, 
or any part of the system that might fail and could 
be repaired in flight. Thus, a properly trained oper- 
ator may be able to overcome a failure in the system 
without having to abandon the mission and return 
to base. (Indeed, getting home may be a serious prob- 
lem in the event of radar failure.) 

Since an ASV system has several functions, the 
number of knobs and switches to control all the func- 
tions will naturally be greater than on a simple radar 
mapping system. This brings up the problem of panel 
layout. All too often, the panel layout is determined 
by the whims of the man who makes the first bread- 
board model of a new system, thus depending only on 
electrical convenience. Later, various modifications 
may be made to facilitate production, but very sel- 
dom are the feelings of the man who will have to oper- 
ate the set considered. It is seldom possible to take a 
poll of all prospective operators, but some thought on 
the part of the designer will indicate such facts as the 
following. (1) Controls most frequently used should 
be the easiest to reach. (2) Controls performing the 
same general function should be grouped together, in 
so far as possible. 

Since a large part of ASV work consists of long 
over- water flights, careful thought must be given to 
navigational aids when planning what equipment the 
aircraft will carry. Since radar beacons are a valuable 


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22 


DESIGN CONSIDERATIONS 


aid to navigation, all ASV sets should incorporate 
beacon equipment, for the additional weight is small 
compared with the value gained. It is essential, how- 
ever, that the beacon part of the ASV set be highly 
reliable, for if an operator attempts to pick up a bea- 
con and fails to do so, he may interpret this failure to 
mean that he is out of range of the beacon rather than 
that the system is not functioning properly. Thus, he 
may become more confused as to his position if the 
beacon part of the system fails than if he had never 
looked for beacons. 

On long search missions the task of the radar oper- 
ator is very arduous, for the success of the mission 
depends on a constant watch being kept on the scope. 
It is therefore important in engineering the ASV sys- 
tem that everything possible be done to reduce oper- 
ator fatigue. Along these lines, the proper lighting of 
meters and dials, and the proper color of the oscillo- 
scope fluorescent screen and filter should be con- 
sidered. 

4.2.2 Installation Design 

Two factors of particular importance in the in- 
stallation of ASV systems are the fatigue and safety 
of the operator and convenience of maintenance. As 
mentioned above, ASV operators have a difficult task 
to perform. The importance of giving a man a com- 
fortable, uncramped position in which to sit is obvi- 
ous as the first step in minimizing fatigue. The proper 
lighting of the compartment in which the ASV oper- 
ator is to work is correspondingly important. A 
blacked out compartment or cubbyhole should be 
provided and it should not be necessary to use a visor 
on the scope continuously. The visor forces the oper- 
ator to sit in just one position, often a cramped posi- 
tion, while looking at the scope. Also, whenever the 
operator looks away from the scope to rest his eyes, 
he loses his dark adaption (unless he happens to be on 
a night patrol). Finally, it is psychologically advis- 
able to provide the operator with a window which, 
though normally blacked out, will permit him to look 
out of the plane if he so desires. This will relieve any 
mental stress due to a feeling of being completely 
closed off from all reality. 

It has been found that electrical interference often 
causes spots not unlike signals to appear on the PPI. 
Presence of such interference from blowers, turrets, 
and generators places the operator under the contin- 
ual strain of attempting to distinguish real targets 
from false ones produced by mechanisms within the 


airplane. When making a radar installation, great 
care should be taken to minimize such interference. 

Since ASV aircraft make attacks from a relatively 
low altitude, and therefore, because of enemy action, 
may have to ditch on very short notice, everything 
possible should be done in making the installation so 
that the operator may quickly get into a safe ditching 
position. That is, an area of the forward bulkhead of 
the ASV compartment should be left clear and this 
area should be easy to reach. Thus, in an emergency, 
the operator can quickly get into a position with his 
back against the bulkhead . Perhaps the best solution 
of this problem is to place the operator’s chair and a 
headrest against the forward bulkhead. 

4.2.3 A Typical System, AN/APS -30 

One of the last ASV systems designed during 
World War II and therefore incorporating most of 
the recent improvements is the AN/APS-30 series. 



Figure 1A. Control unit (AN/APS-30). 

The series consists of four systems with modulator, 
indicators, synchronizer, stabilization equipment, 
and cabling common to all. Two of the systems, the 
AN/APS-32 and the AN/APS-34, were designed 
primarily for bombing, resolution being stressed at 
the expense of range; they will not be further dis- 
cussed here. The other two systems are the AN/APS- 


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ENGINEERING CONSIDERATIONS 


23 



Figure IB. Plan position indicator (AN/APS-30). 



Figure 2 A. R-F head (AN/APS-30). 

31 and the AN/APS-33. The AN/APS-31 has a small 
antenna which scans only through 150 degrees in the 
forward direction, while the AN/APS-33 has an an- 
tenna with a 29-in. aperture scanning through 360 de- 
grees. The major components of the latter system are 
shown in the accompanying photographs (see Fig- 
ures 1, 2, 3, 4). The components for the AN/APS-31 
are identical except for the scanner assembly. 



Figure 2B. Modulator (AN/APS-30). 

The series incorporates several improvements over 
previous systems. All the controls have been grouped 


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24 


DESIGN CONSIDERATIONS 


in one position and the control unit and the indicator 
are small enough so that they may be mounted prac- 
tically anywhere, thus simplifying the installation 
problem and permitting a physically satisfactory in- 



Figure 3. Synchronizer (AN/APS-30). 


stallation. It is of interest to observe that, although 
there has been some grouping of controls on the con- 
trol unit itself in relation to their functions, the gen- 
eral principle of the layout of the control unit is still 
the principle of symmetry. Since the sweep and gain 
controls are those most frequently used by the oper- 
ator, undoubtedly it would have been better to have 
the sweep control in the lower left-hand corner, which 
is a position easier to reach than the center of the box. 

Another large improvement is in the beacon fea- 
tures that the system incorporates. Beacon automatic 
frequency control, a wide band intermediate frequency 
[i-f] amplifier, and video stretching all combine to 
improve beacon performance greatly. A delayed fast 


sweep permits reading the beacon coding at long 
ranges. 

Typical ASV features are the target discrimination 
[TD] control which permits examining a target on a 



Figure 4. AN/APS-33 scanner assembly. 


fast sweep while it is still at long range, thus aiding 
differentiation between clouds and more solid targets, 
and the provision of a 5-jusec pulse on long-range 
sweeps. 

A final feature, which is to be desired in any radar 
system, is the hermetically sealed r-f head and modu- 
lator, which includes the indicator high-voltage 
supply. 


ISTRICTED , 


Chapter 5 

ASY PERFORMANCE 


The three foregoing chapters have considered the 
functions of, means of detection by, and design con- 
siderations for aircraft to surface vessel [ASV] radar 
systems. It is now desirable to examine the perform- 
ance which can be obtained with an ASV radar 
system, with particular reference to the various fac- 
tors affecting that performance. In general three 
major factors determine the performance of an ASV 
system. The first, radar maintenance, is directed at 
attaining and maintaining the peak performance for 
which the particular ASV system was designed; this 
is analogous to maintenance of the aircraft motors to 
insure optimum performance. The second factor, 
range performance, is associated with the operational 
features influencing maximum range, such as horizon 
limitations on range, and the significance of scanning 
rates and the observed persistence of targets on the 
indicator screen. The third factor, operational meth- 
ods, involves the relationship between tactics and 
performance, and includes operator fatigue, methods 
of patrol and attack, and target identification (as 
friend or foe). These three groups of factors are dis- 
cussed in the three sections that follow. Although 
this discussion pertains to ASV performance, certain 
of the concepts are directly and equally applicable to 
the performance of other radar systems discussed in 
subsequent chapters. This is particularly true of the 
section on radar maintenance. Sections 7.4 and 15.2.2 
are an extension of the application of these notions to 
bombing and aircraft interception [AI] radar systems 
respectively (essentially similar extensions can be 
applied to airborne gun laying [AGL] and airborne 
moving target indication [AMTI] systems discussed 
in Parts IV and V). 

5.1 RADAR MAINTENANCE 

5.1.1 Introduction 

The function of radar maintenance is to attain and 
maintain peak performance of the radar system. 
When two different radar sets are compared on the 
basis of the operational ranges obtainable with each, 
the set with the greater range is said to have the 
higher performance. This type of performance, called 
range performance, depends upon the design char- 
acteristics of the radar system including such fea- 
tures as the gain of the antenna, scanning rates and 


scanning losses, antenna beamwidth, the type of in- 
dication, the peak power output of the transmitter, 
and the sensitivity of the receiver. 

The concept of performance used in radar main- 
tenance is called radar performance. This concept dif- 
fers from the usual concept of range performance. Its 
definition and some general considerations concern- 
ing it are given in Section 5.1.2. Other aspects of 
radar maintenance, namely, maintenance policy, de- 
sign considerations, instruction literature, mainte- 
nance training, and special radar test equipment are 
presented in Sections 5.1.3 to 5.1.7. 

5.1.2 General Considerations 

Radar performance is defined as the ratio of the peak 
power output of the transmitter to the weakest signal 
power which the receiver can detect : P/p mm . This defi- 
nition is modified somewhat for the purpose of tests 
and measurements in that the signal power used is a 
test signal power, p T . Hence, the definition of radar 
performance (for maintenance purposes) becomes 
P/Vt- 

Usually Pr is selected as one of the following three quantities : 
( 1 ) the minimum discernible test signal power, which may be 
the same as the minimum discernible signal power; (2) the 
tangential test signal; and (3) the continuous wave [CW] test 
signal power equal to the noise signal power. A minimum dis- 
cernible test signal is an amplitude-modulated (pulsed) or 
frequency-modulated signal which is attenuated (through at- 
tenuators in the test set) until it is barely discernible in the 
receiver noise. A tangential test signal is an amplitude- or 
frequency-modulated signal which is attenuated until the 
bottom of the noise on the signal pattern is of the same height 
as the top of the noise without the signal. A CW test signal is 
attenuated until the voltage developed across the second de- 
tector (as measured with a meter) indicates a power equal to 
the noise power (as measured with a meter in the absence of 
signal). 

The ratio P/pr, expressed in decibels, is called the 
radar performance figure, and is a measure of the abil- 
ity of the radar system to detect targets with a given 
set of external conditions. Assuming constant ex- 
ternal conditions, for example a target in free space, 
it is possible to correlate changes in range with 
changes in radar performance by using the inverse 
fourth power law. Since the power of a returning sig- 


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25 


26 


ASV PERFORMANCE 


nal is always proportional to the power in the trans- 
mitted pulse this law may be written 



where Pr is the power received from a target at 
range R, P is the peak transmitted power, and K is a 
constant depending upon the characteristics of the 
target and design factors of the radar system [see 
equation (1) of Chapter 7 and equation (3) of Chap- 
ter 15]. For beacon reception the inverse square 
power law obtains, namely, 

R = K(P/ Vt )\ 

If the relative value of range is plotted in per cent 
against the decrease in radar performance figure, a 
graph such as that shown in* Figure 1 is obtained. 

A priori, the range performance of a radar system 
is judged on the basis of the maximum range obtain- 
able with it. This, of course, is necessary for opera- 



Figure 1 . Radar performance in decibels as a func- 
tion of range. A = radar curve, B = beacon curve. 

tional comparisons. However, it is not possible to 
measure radar performance accurately by observa- 
tion of the maximum range, since in general, com- 


plete information on the external factors affecting 
range is not available. The external factors which 
affect range are: (1) the reflection properties of the 
target; (2) differences in path length between re- 
flected rays, which may cause the rays to reinforce or 
to cancel; and (3) atmospheric conditions, such as 
temperature and humidity effects which may cause 
the radar beam to be bent upward, thus rapidly dissi- 
pating the energy in the beam, or downward, thereby 
delaying the dissipation of the energy. A change in 
any one of these conditions is reflected as an apparent 
change in radar performance, which may be favorable 
or unfavorable — an apparent increase or decrease in 
performance. Consequently, in judging the perform- 
ance of a radar system, it is unwise to use the signal 
returned from a given target selected as a standard. 
Rather, the radar performance figure should be meas- 
ured by use of the appropriate test equipment. 

The internal factors (those within the radar set) 
which influence radar performance are of two kinds: 
those affecting peak transmitter output power and 
those affecting the minimum discernible signal. The 
peak power output of the transmitter is dependent 
upon the following: (1) the quality of the transmitter 
tube (spectrum and tube efficiency); (2) the mag- 
netic field strength (if a magnet is used in conjunction 
with the transmitter tube such as with magnetrons) ; 
(3) the pulse peak voltage applied to the transmitter 
tube ; (4) the shape of the pulse voltage applied to the 
transmitter tube; (5) losses or mismatch in the trans- 
mission line, caused by a detuned TR or foreign ma- 
terial or discontinuities in the line; (6) a nonfiring 
TR or ATR; and (7) losses in the rotating joint or 
antenna connections. In practice, the average power 
output of the transmitter is measured and correlated 
with the peak power output by the relation 

P avg = P ’ T ’ V r i 

where P avg is the average power output of the trans- 
mitter, r is the pulse duration, and v r is the pulse re- 
currence frequency. Thus, the pulse duration and the 
pulse recurrence frequency have to be known in order 
to determine peak power output accurately. 

The receiver performance figure, 10 log pr, which 
is a measure of the ability of the receiver to detect a 
weak signal as determined by a test signal pr is de- 
pendent upon the following: (1) quality of the crystal 
(gain and noise); (2) local oscillator tuning; (3) auto- 
matic frequency control [AFC] performance; (4) TR 
and ATR tuning, and losses through each of these; 
(5) noise of local oscillator; (6) noise of the i-f ampli- 


RESTRICTED 


RADAR MAINTENANCE 


27 


fier; (7) transmission line losses; and (8) the factors 
which determine how far into noise a signal can be 
seen, namely, i-f bandwidth, video bandwidth, pulse 
recurrence frequency, sweep speed, spot size, and 
type of presentation (if an oscilloscope is used, as is 
generally the case in making receiver sensitivity 
measurements) . 

Receiver sensitivity is best measured in terms of 
noise figure, which is accurately defined but some- 
what difficult to measure, especially under field con- 
ditions. 

Noise figure is the ratio of the available noise power from all 
sources within the actual receiver including the antenna (re- 
ferred to the input terminals) to the available noise power from 
the antenna alone ( k°TB where k° is Boltzmann’s constant, T 
is the absolute temperature, and B is the i-f bandwidth). Noise 
figure is usually expressed in decibels and is equal to unity 
for an ideal receiver. The test signal power pt in decibels is 
related to the noise figure as follows. 

10 log pt = 10 log k°KB — N r + C, 

wiiere N r is the overall receiver noise figure and C is a constant 
depending upon the i-f bandwidth, the video bandwidth, the 
pulse recurrence frequency v r , the sweep speed, spot size on the 
indicator, and type of presentation. C is determined empiri- 
cally; it is negative for a minimum discernible test signal 
(approximately 4 to 8 db below the signal whose power is just 
equal to the noise power); and positive for a tangential test 
signal (of the order of 6 to 10 db above the signal whose power 
is just equal to the noise powder). 

In practice the value obtained for the test signal 
power is taken as a measure of the receiver sensi- 
tivity; the precise value to be attained is prescribed 
for the particular system on which the measurement 
is made, and for the type of test signal employed. A 
check on the i-f bandwidth can be made with an 
amplitude or frequency modulated test signal. 

Since the radar performance figure is determined 
by the peak power output of the transmitter and the 
sensitivity of the receiver, each of these quantities 
should be measured and the radar performance figure 
computed. The adequacy of performance is deter- 
mined by comparing the value obtained in this way 
with the prescribed value for the radar performance 
figure. Some of the factors affecting the peak power 
output of the transmitter and the sensitivity of the 
receiver are not easy to check in the aircraft. There- 
fore, in the process of maintaining the peak perform- 
ance of the radar system, it is necessary to specify the 
factors which are to be checked in order to determine 
the radar performance figure and guarantee peak 
performance; where the various factors are to be 
measured, for example whether at the aircraft or on 


the bench; and the relative frequency with which the 
measurements are to be made. In order to insure 
rapid maintenance with the greatest efficiency, a spe- 
cific maintenance procedure (maintenance policy) 
should be adopted and pursued. The basic considera- 
tions involved in such a policy are presented below. 
Of course, the evolution of various types of radar 
systems, improvements of engineering design, and 
installations in new types of aircraft will necessitate 
a modification of these considerations. 

5.1.3 Maintenance Policy 

A maintenance policy that proved useful and suit- 
able during World War II was based upon the follow- 
ing practices: (1) making a brief series of quick, sim- 
ple, routine tests at the aircraft to check the per- 
formance of the system before each flight, (2) making 
a series of simple tests at the aircraft to localize a 
faulty major assembly so that the faulty assembly 
can be replaced; and (3) making a series of tests on 
the bench for the isolation of a faulty subassembly or 
functional unit in a major assembly, so that it may be 
replaced and subsequently repaired. 

When a maintenance policy of this kind is adopted, 
two facts have to be known: (1) the parameters 
which have to be measured in each of the above three 
categories have to be specified; and (2) terminals 
(called test points) for connecting the test equipment 
to the radar system have to be provided. The param- 
eters can be grouped conveniently into two classes : 
those necessary for making routine measurements 
and for isolating one faulty major assembly from an- 
other; and those necessary for isolating one faulty 
subassembly or functional unit from another. Corre- 
spondingly, the test points can be classified as ex- 
ternal test points — those essential for making the 
first and second categories of measurement; and in- 
ternal test points — those essential for making the 
third category of measurements. Since the inclusion 
of test points in a radar system affects the design of 
the system, test points are discussed somewhat more 
fully under the subject of design considerations (see 
Section 5.1.4). 

The parameters to be checked in making routine measure- 
ments were arbitrarily designated primary parameters; those 
for isolating one faulty major assembly from another, sec- 
ondary parameters; and those for isolating one faulty sub- 
assembly or functional unit from another, tertiary parameters. 
Some of the primary parameters that were used are (1) re- 
ceiver sensitivity, on search and beacon; (2) transmitter 
power output, on search and beacon; (3) AFC operation; 


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28 


ASV PERFORMANCE 


(4) TR recovery (primarily for systems requiring close-in per- 
formance); (5) transmitter tube and crystal currents; and 
(6) transmitter spectrum. Some of the secondary parameters 
measured were (1) ground continuity between all major as- 
semblies, open circuits from ground, and continuity of all cable 
leads; (2) d-c and a-c voltages; (3) sweep voltages; (4) choke- 
flange joints in the wave guide for secure fits. Some of the 
tertiary parameters included filament voltages, critical volt- 
ages and waveforms, critical currents, and AFC and local 
oscillator adjustments. 

5.1.4 Design Considerations 

Execution of the above maintenance policy im- 
poses specific requirements on the design of the radar 
system. The light weight and small size of the equip- 
ment is fundamental for aircraft radar systems. The 
design has to be such that the system as a whole is 
comprised of individual major assemblies which can 
be readily isolated one from another in the event that 
one is faulty; and such that each major assembly is 
comprised of individual subassemblies (in so far as 
practicable), which also can be readily isolated one 
from another in the event that one is faulty. A ju- 
dicious compromise should be effected between the 
requirements for ease of maintenance (sufficiency and 
accessibility of test points) and the necessity for light 
weight and stowage in remote corners of an aircraft. 

Test points are designed into the radar system. Ex- 
ternal test points for measuring the primary param- 
eters are located on a readily accessible test panel. 
External test points for isolating one faulty major 
assembly from another are on the major assembly 
proper and are designed for accessibility when the 
equipment is installed. Internal test points, for iso- 
lating faulty subassemblies or functional units, 
should be located on a terminal board used as a test 
panel, or in some cases on the subassemblies or func- 
tional units themselves. 

Essential external test points are: (1) directional coupler 
output, (2) trigger output, (3) test video (including range 
marks), (4) d-c and a-c input voltages, (5) scanner on-off 
switch, and (6) critical voltages and waveforms (should be 
limited in number). Essential internal test points are: (1) a-c 
and d-c voltages, (2) currents (including crystals and trans- 
mitter tube currents), (3) impedances, and (4) points for ex- 
amining critical waveforms, including range marks. 

Test points should have certain characteristics, in- 
cluding ready accessibility, appropriate and clear 
labels, standard connectors or fittings, and presenta- 
tion of the system parameter to be tested in an ap- 
propriate form (with regard to voltage and power 
level) for use with the available test equipment. 


Recommendations as to the characteristics of these 
various test points have been made by the Joint 
Radio Board and are contained in its reports . 8-10 

The most important external test point is the out- 
put of the directional coupler, for without this test 
point it is not possible to make simple, reliable, quan- 
titative measurements of the fundamental quantities 
in determining radar performance, power output, and 
receiver sensitivity. 

A directional coupler is a device by which a known fraction 
of r-f energy is coupled out of the main transmission line (wave 
guide or coaxial) to a power meter, a frequency meter, an echo 
box, a signal generator test set, or a spectrum analyzer; and 
by which this same known fraction of energy is coupled back 
into the transmission line from the echo box or from the signal 
generator test set. A directional coupler consists of a short 
section of transmission line (usually part of the main line) and 
an attached section of secondary line (called the auxiliary line). 
One or more openings (holes or slots) couple energy from the 
main line into the auxiliary line which is then coupled out 
through a matched probe to the test point, which may be a 
coaxial fitting or a wave guide fitting. Some couplers are uni- 
directional, coupling energy from the transmission line from 
one direction only; others are bidirectional, coupling energy 
from the transmission line from either direction. The desirable 
characteristics of a suitably designed directional coupler are 
contained in reports 8-10 of the Joint Radio Board. 

A pick-up antenna (horn or dipole) placed in front 
of the radar system antenna provides for connecting 
the radar system to the test equipment, such as the 
signal generator test set, power meter, echo box, or 
frequency meter. It is only by use of a pick-up an- 
tenna that a check on the entire radar system, includ- 
ing the antenna, can be made. Unless the coupling 
loss between the system antenna and the pick-up 
antenna is accurately known, precise measurements 
cannot be made. There are some systems for which it 
is desirable to use a pick-up antenna for alignment 
procedures, such as for boresighting (alignment of 
the axes of the guns and the radar system antenna) 
or antenna alignment in general, in which case it is 
not necessary to know the magnitude of the coupling 
between the two antennas. Hence, an additional 
aspect of radar system design (from the point of view 
of radar maintenance) becomes important, namely, 
ease in removing the radome. Thus, access to the 
scanner is provided, for possible adjustment or repair. 

The design of the radar system may be influenced 
also by the inclusion of special test accessories, such 
as built-in test oscilloscopes for some long-range navi- 
gational systems or a calibrated movable range mark 
for echo box measurements. 


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29 


5.1.5 Instruction Manuals and 

Maintenance Training 

Once the maintenance policy has been established 
and the radar system designed in accordance with the 
tenets of such a policy, literature for maintenance 
personnel should be prepared. During World War II 
this literature took the form of instruction manuals, 
called “Handbooks of Operating and Maintenance 
Instructions.” These handbooks usually described 
the entire radar system with a separate section on the 
maintenance of that system. The sections on main- 
tenance in the handbooks which appeared during the 
early part of the war were often not very complete, 
because of the state of radar development and the in- 
herent lag between system and test equipment design 
and production. However, the handbooks prepared 
toward the close of World War II were very compre- 
hensive indeed. 

The following is a list of the features included in the section 
on maintenance in one of the recent handbooks : ( 1 ) the precise 
maintenance procedure to be followed; (2) a complete descrip- 
tion of and operational procedures for the use of each item of 
test equipment, including all the requisite calibration data; 
(3) the rated and minimum acceptable values for the various 
parameters; (4) a list for and relative frequency of routine 
inspections, such as daily inspection, 100-hour inspection, and 
500-hour inspection; (5) removal, disassembly, and servicing 
of various units; (6) alignment procedures for the radar system; 
(7) complete circuit diagrams for the radar system (major 
assemblies and subassemblies) and for the test equipment to 
be used with it; (8) a trouble-shooting chart for the isolation 
of one faulty major assembly from another; (9) a trouble- 
shooting chart for isolation of one faulty subassembly or 
functional unit from another; (10) a trouble-shooting chart 
for the localization of r-f troubles; (11) a trouble-shooting 
chart for localizing troubles in faulty subassemblies; (12) a 
complete list of voltages, currents, impedances, and wave- 
forms to be expected with the precise method of measurement 
given; (13) a list of the functions of each major assembly; (14) 
a list of the tube complement and function; and (15) a list of 
emergency repairs, which might be made in flight. 

Adequately trained maintenance personnel are es- 
sential to the attaining and maintaining of peak radar 
performance with the greatest efficiency. Although 
this statement is self-evident, the problems associ- 
ated with a suitable training program are very ex- 
tensive indeed. A detailed discussion of these prob- 
lems is presented in Section 13.2.3. 

5.1.6 Radar Test Equipment 

The rather amazing development and applications 
of radar systems during World War II was accom- 
panied by an equally amazing development of test 


equipment for radar systems. Since the radar per- 
formance figure can be determined accurately only 
by use of appropriate test equipment, it is of interest 
to note some of the items of test equipment that were 
developed during the war for use with radar systems. 
The following is a list of a few such items, with their 
essential functions. Detailed listing of particular 
test equipment items for individual radar systems is 
contained in the recommendations of the Joint Radio 
Board 8 ; descriptions and properties of the items 
detailed in the Joint Radio Board recommendations 
(and for other test equipment items) are presented in 
the U. S. Radar Survey. 1 

1. Echo Box. Provides a quick, rough measure of the 
overall radar performance. It measures frequency, relative 
power (roughly), and spectrum width; detects double moding 
of transmitters and checks on AFC operations. The echo box 
is not suitable for beacon receiver sensitivity checks. 

2. Power Meter. Measures the average power of radar 
transmitters, and of CW or modulated signal generators. It 
can be used in conjunction with an echo box to localize poor 
overall radar performance in the transmitter or in the receiver. 

3. Frequency Meter. Measures the frequency of radar trans- 
mitters, CW or modulated signal generators, and beating 
oscillators; detects double moding; and in some cases con- 
tains provisions for viewing the pulse or wave shape. In 
general, it is not essential to measure radar transmitter 
frequency; also the output of certain local oscillators is inac- 
cessible. The most useful application is for checking signal 
generator output at beacon frequency; consequently, con- 
siderable absolute accuracy is required ( ± 0.5 me). 

4. The Signal Generator Test Set. Measures average trans- 
mitter power on search and beacon; receiver sensitivity on 
search and beacon; checks AFC operation and local oscillator 
adjustments; measures transmitter frequency, spectrum width 
(if the Q of the frequency meter is sufficiently high); checks 
TR and ATR tuning, TR recovery, and nonfiring TR’s and 
ATR’s. 

5. Crystal Checker. Measures front resistance, back re- 
sistance, and back crystal current at 1 volt. On the basis 
of these measurements good crystals can be selected from poor 
ones. 

6. Radio-Frequency Load. Provides an r-f test load of good 
match, into which the transmitter can operate. 

7. Standing Wave Device. Measures voltage standing wave 
ratio of r-f components in wave guide and transmission lines 
and is particularly useful for very high frequency radar trans- 
mitters which are frequency sensitive to the matching load 
into which they operate. 

8. Spectrum Analyzer. Displays a picture of all frequencies, 
within a given band radiated by the transmitter and local 
oscillator; measures pulse duration, spectrum width, and the 
Q of resonant cavities, and checks the frequency of signal 
generators, local oscillators, transmitters, TR and ATR boxes. 

9. Power Absorption Cone or Screen. Absorbs power from 
the antenna (to prevent reflections from nearby objects and 
interference with nearby systems). 


I 


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ASV PERFORMANCE 


5.1.7 Conclusion 

The significance of radar maintenance in ASV per- 
formance has been discussed above in some detail 
both because the subject is important and because 
the concepts involved differ somewhat from many of 
those discussed in other parts of this book. It is of 
value to reiterate the earlier statement that although 
this section on radar maintenance was written pri- 
marily with reference to ASV performance, the con- 
cepts involved are immediately applicable to other 
types of radar systems, with modifications and ex- 
tensions in accordance with the requirements of the 
particular system. 

5.2 RANGE PERFORMANCE OF A 
TYPICAL SYSTEM 

One of the most important characteristics of any 
ASV radar is its maximum range. Many factors affect 
the range of a radar set. 4c One limiting factor that 
a microwave radar set cannot overcome is the 
horizon. 

5.2.1 Horizon Limitations 

In Table 1, the horizon range is given for several 
heights of the radar antenna and the reflecting ob- 
ject. Various cases of anomalous propagation have 
been observed on ground-based or shipborne radars, 2a 
but airborne radars have not given a greater than 
horizon range, except for the very slight increase in 
range due to refraction of the radar beam in the at- 
mosphere. Thus, Table 1 shows the maximum range 
that may be expected from a microwave radar set 
under the conditions shown. 


Table 1 . Approximate horizon range in nautical 
miles for various target and radar heights. 


Height of radar 
in feet 

5 

Height of target in feet 
10 20 30 

50 

100 

15.1 

16.2 

17.8 

19.0 

21.0 

300 

25.9 

27.0 

28.6 

29.9 

31.2 

500 

30.3 

31.4 

33.0 

34.2 

36.2 

1,000 

41.6 

42.7 

44.3 

45.5 

47.5 

3,000 

70.1 

71.2 

72.8 

74.0 

76.0 

5,000 

89.7 

90.8 

92.4 

93.6 

95.6 

10,000 

102.8 

103.9 

105.5 

106.7 

108.7 

30,000 

215.8 

216.9 

218.5 

219.7 

221.7 

50,000 

277.8 

278.9 

280.5 

281.7 

283.7 


From Table 1 it will be seen that even for an air- 
craft at 10,000 ft, the horizon range is still less than 
150 miles. With present beacon techniques, reception 
at 150 or even 200 miles is not exceptional. Therefore, 


since ASV search missions are seldom carried out at 
altitudes greater than 10,000 ft, it is safe to say that 
beacon reception will only be limited by the horizon. 
The same cannot be said for radar ranges. 

5.2.2 Typical Values 

Before mentioning the maximum ranges that may 
be expected with certain ASV systems on various- 
sized ships, it is necessary to define what is meant by 
the maximum range of a radar set. Since it is cus- 
tomary for an ASV system to scan at a rate suffi- 
ciently slow so that the eye can easily resolve the 
separate scans, the observer will notice that as the 
range to a target increases, the signal decreases in 
intensity; but before the intensity is so low that the 
signal is indistinguishable from noise, he will note 
that although the signal may have fairly high inten- 
sity on a certain scan, it will not appear at all on the 
subsequent scan. As the range continues to increase, 
the percentage of scans in which the target is visible 
will continue to decrease until the operator may feel 
fairly certain, but still cannot be positive, that the 
target will not appear on the next scan. At such a 
point, the radar system could be considered to have 
reached its maximum range on the target in question. 
On the other hand, it is the purpose of an ASV system 
to search the entire area about the plane that carries 
the system. An operator can hardly be expected to 
notice every signal that appears on the PPI every 
scan. Thus, if the range to a certain target is so great 
that the signal appears only once every six scans, it is 
very possible that the operator will completely over- 
look that target. Since it now becomes evident that 
the useful maximum range of an ASV system depends 
to a large extent on the vigilance of the operator, it is 
necessary to be somewhat arbitrary in defining maxi- 
mum range. Therefore, the term maximum range as 
used in this section will mean the range at which the 
target is visible on 50 per cent of the scans. 

Although with the older ASG systems such as the 
SCR-517 or the ASG a range of 50 or 60 miles on a 
large battleship would be considered good, a recently 
designed system such as the AN/APS-33 is capable 
of horizon range on the same type of ship unless the 
ASV system is above 10,000 ft. Cruisers or large 
freighters give a maximum range of 60 to 70 miles on 
the AN/APS-33, whereas destroyers and smaller 
freighters give ranges of 40 to 50 miles. 

Submarines will give a maximum range of approxi- 
mately 30 miles when riding high in the w^ater, but 





OPERATIONAL METHODS 


31 


when running partially submerged, or completely 
submerged with only a periscope or a Schnorkel tube 
above the surface, the maximum range is greatly re- 
duced. Under such conditions, special means (see 
Section 3.2.3) must be resorted to in order to obtain 
any reasonable sort of range. A system designed par- 
ticularly for detecting submarines operating with 
only a Schnorkel tube exposed, the AN/APS-15BM, 
incorporating a short pulse (1/4 n sec), narrow beam 
(1.4 degrees), and the latest anti-sea-return circuits, 
showed a maximum range of 14 miles on a Schnorkel. 
Systems less specially designed have ranges of 8 to 
10 miles on a Schnorkel. 

5.3 OPERATIONAL METHODS 

5.3.1 Fatigue 

The task of watching a scope hour after hour is a 
very difficult one, and many of the ASV aircraft used 
during the war were capable of patrolling for 18 hours 
or more. Tests indicate that a period of 3^ to 1 hour 
is as long as a man can operate an ASV system and 
still maintain high efficiency. Eyestrain is particu- 
larly bad in the case of a PPI, where the operator 
must follow the rotating trace with his eyes in order 
to have the best chance of seeing weak signals. 

Fatigue can be moderated in four-engined aircraft, 
which normally carry a crew of nine to twelve men, 
by rotating watches. In the RAF four men were 
trained so that they could trade shifts at the radar, 
the radio, and the guns. Twin-engined aircraft nor- 
mally have a large enough crew so at least two men 
can switch positions hourly. Single-engined planes 
of the TBM type present more difficulty in alternat- 
ing positions, and indeed in some planes of this gen- 
eral class positions cannot be changed in flight. How- 
ever, most of the search radar equipments built dur- 
ing World War II were equipped with at least two 
indicators, and the auxiliary indicator can be in- 
stalled so that another crew member may relieve the 
radar operator without changing positions. The bur- 
den of keeping the radar in proper adjustment is still 
left to the radar operator in this type of installation. 

Since watching a PPI is a particularly tiring task, 
some work has been done on circuits which are de- 
signed to flash a light or provide an audible tone 
when a target appears between previously set range 
limits (alarm circuits). A really good alarm system 
would be of very great help to an operator by sup- 
plementing his watch of the tube. 

Early radars of the lobe-switching type employed 


a double A scope for indication (see Section 3.1.2). 
This type of indication does not require the operator 
to move his eyes as does the PPI. Operators report 
that this is a very big advantage on long patrols. 
Possibly some such type of indication could be em- 
ployed on microwave radars as an auxiliary to the 
PPI. An A scope employing a tube with a persistent 
screen, with the signals intensity-modulating (as well 
as deflecting) the electron beam, is worth investigat- 
ing as a means of relieving operator eyestrain. 

5.3.2 Patrol Methods 

Three general types of operations were carried on 
during the war by ASV planes: patrol sweeps over 
large areas such as the Bay of Biscay, convoy escort, 
and the hound-to-death search of a small area. Each 
of these has special problems associated with it. 

The sweep of an area is the most straightforward 
type of search. Here a group of planes are sent out to 
fly parallel paths through an area, usually returning 
through the area on another set of parallel courses. 
The principal problem is that of navigation, insuring 
that all parts of the area are covered without a large 
amount of overlapping. The number of planes to 
search an area completely is, of course, inversely pro- 
portional to the dependable range of the radar system 
employed. A radar system which gives maximum 
ranges of 30 miles broadside and 15 miles end-on for 
a submarine, taking into consideration the possible 
distribution of aspects and other factors, gives a 
swept path (a path width in which all submarines 
would be seen) of about 12 miles. To sweep an area 
such as the Bay of Biscay which contains about 
150,000 square miles requires a considerable number 
of airplanes employing the best of the microwave 
radar developed during the war. 

Increasing the range of the radar does not gain 
operationally all that one might expect. Targets 
picked up on the radar must be investigated. Good 
identification of friend or foe [IFF] would eliminate 
the need for investigating friendly vessels. However, 
with present radar systems, a great many targets 
turn out to be such things as whales, porpoises, and 
floating debris. If targets are sighted at long range, 
the patrolling aircraft must depart a long way from 
its course to investigate them. Thus, there will be 
some optimum system performance for a particular 
area, considering that increased performance is gen- 
erally obtained at the cost of increased weight and 
increased drag. In an area where there are relatively 


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ASV PERFORMANCE 


few targets other than those sought, a long-range 
system will be useful. In a relatively congested area, 
little would be gained by going to higher-powered 
systems. 

Many planes were lost during World War II be- 
cause of errors in navigation. Flying 8 or 9 hours out 
to sea and back again with deviations from the patrol 
course to investigate targets puts a heavy burden on 
the navigator. Loran becomes of great value under 
these conditions. 

Long-range planes, operating from Newfoundland, 
Iceland, and Northern Ireland were called upon to do 
much convoy work reaching out into mid-Atlantic. 
One of the major problems here was that of locating 
the convoy especially in bad weather. A long-range 
microwave radar set was a great help, although pilots 
frequently flew too low to get maximum ranges. 
Microwave beacons carried by one of the escort 
vessels, or Loran, would have been valuable aids. 

Once the convoy was located, the planes circled it 
hour after hour, always on the lookout for subma- 
rines. In the early days of the war, when escort ves- 
sels were scarce, the escorts did not dare leave the 
convoy to attack a submarine for fear another would 
slip in while they were gone. The submarines would 
attempt to slip by the convoy on the surface during 
the day in order to lie in wait ahead of them at night. 
The escort planes did a very good job of keeping the 
submarines submerged, and since the submarines’ 
underwater speed was less than that of the convoy, 
they could be left behind. 

Planes flying from escort carriers and equipped 
with radar did a most effective job in convoy pro- 
tection. Some of these planes were also equipped for 
night operation. The principal weakness remaining 
in the defense was the difficulty of flying from small 
carriers in rough seas and poor visibility conditions. 
Land-based planes also frequently failed to make an 
appearance because their bases were closed in by 
weather. Extension of all-weather flying techniques 
would relieve this situation. 

The hound-to-death tactics could be made very 
effective. These tactics are employed once a subma- 
rine has been detected but not killed. The submarine 
skipper, when detected, has very few choices of tac- 
tics to be employed. He can sit on the bottom, if the 
water is shallow, and wait for possibly 60 hours, or 
can decide to proceed at the most economical under- 
water speed for maximum range. (The German sub- 
marines could cover about 40 miles at a speed of 
2 knots.) Thus, after a contact and a dive, the area in 


which the submarine can be is initially very small, 
expanding slowly until the radius is 40 miles. If the 
submarine is sighted when it surfaces after a pro- 
longed submergence and is forced to dive again at 
once, then its defensive measures are almost ex- 
hausted. A sufficiently intensive search in the area is 
bound to result in a kill. 

One of the biggest difficulties in employing these 
tactics is getting an accurate fix at the time of sight- 
ing. Loran would of course help here. Another possible 
approach would be to have the sighting craft, plane 
or surface vessel, drop a floating microwave beacon. 
This could be homed on either by plane or sur- 
face vessel. Sono buoys dropped by the aircraft could 
also be a big help. During the war, submarines fre- 
quently made their escape because of the extreme 
difficulty of keeping the search area fixed, particu- 
larly under poor visibility conditions. Extension of 
navigation and marking techniques and improve- 
ments in all-weather flying abilities will help. 

During the closing days of the European opera- 
tions of World War II, the Schnorkel 7 made its ap- 
pearance. Some Schnorkel installations included very 
effective radar camouflage. This presented a difficult 
problem which was never satisfactorily solved. The 
maximum range on such an object was small, par- 
ticularly in a rough sea. Also the problem of seeing it 
through sea return was great. The attack initiated 
and to be followed in attempting to solve this prob- 
lem includes the following points. 

1 . Increase in maximum range by improving radar 
system performance. 

2. Improvement in signal-to-sea-return ratio by 
shorter pulses, by narrower antenna beams, and by 
circuits to suppress sea return. 

5.3.3 Methods of Attack 

The most common type of attack made by aircraft 
against submarines was a daytime, depth-charge at- 
tack. Depth charges were released from a very low 
altitude of about 30 to 50 ft. These were released in a 
string. A string of six 250-pounders was frequently 
used with 100-ft spacing. The release was usually 
made by the pilot without benefit of bombsight. The 
most desirable attack was a quartering attack. Since 
a submarine is noisy when running on the surface, a 
plane cannot be heard at any great distance from the 
deck. Some pilots were able to take advantage of this 
in making a down-sun approach in order to strike 
with very little warning. With planes which carried 


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SUMMARY 


33 


sufficient depth charges, this sometimes permitted a 
second run. 

A German submarine required about 40 sec to 
crash dive. If the submarine crew spotted the plane 
when it was several miles off, they could dive suffi- 
ciently rapidly to get away. On the other hand, a 
skillful aircraft crew could keep the submarine in 
sight with the radar without being seen from the sub- 
marine, get on a course to give a down-sun approach, 
or in some cases, climb into the overcast, and thus in- 
crease their chances of making an undetected ap- 
proach. It will be seen that such an approach requires 
the closest cooperation between pilot, radar operator, 
and navigator, thus emphasizing the importance of 
continuous training of the whole crew (see Sec- 
tion 13.2.3). 

Many night attacks were made by using search- 
lights on the aircraft. These lights were fitted with 
controls so that they could be steered by a crew mem- 
ber. When a target was picked up by the radar, the 
aircraft started on a homing course without change of 
altitude until the range to target was reduced to 
about 5 miles. At this point altitude was rapidly re- 
duced to 500 ft or less in order to avoid losing the 
target. The approach was continued until the plane 
was about three-quarters of a mile away and the 
light was then switched on. An attempt was made 
to pick up the target in the light and carry on to a 
depth-charge attack. Considerable difficulty was ex- 
perienced in picking up the target with the light. 
This was somewhat alleviated by carefully given 
homing instructions and range information from the 
radar operator. The British developed a radar system 
which automatically directed the light on the target. 

5.3.4 Identification of Friend or Foe 

The reader may have gathered from the previous 
paragraphs that it was not customary to drop bombs 
on a target picked up by ASV equipment unless a 
visual contact was also achieved. Such, unfortu- 
nately, was true. During the early stages of World 
War II, some Allied vessels were not outfitted with 
equipment for identifying friend from foe [IFF] and 
even after all vessels were so equipped, there was a 
continued lack of faith in the infallibility of IFF and 
pilots were required to make visual as well as radar 
contact. (This was not true in those areas considered 
to contain only enemy shipping.) The lack of IFF or 
lack of faith in IFF imposed serious limitations on 


ASV tactics. For example, a plane often had to devi- 
ate far from its patrol course just to look at a friendly 
vessel. Or again, as mentioned above, aircraft in- 
tended for night operations often were loaded down 
with heavy searchlights, thus decreasing either gas 
or bomb load. 

In view of the existence of excellent low-altitude 
radar computers (see Sections 8.4.1 and 8.4.2), the 
handicap imposed on ASV operations by the IFF 
situation becomes obvious. Future ASV planning 
should consider the importance of developing IFF on 
a par with developing good radar. 

5.4 SUMMARY 

As stated before, the primary function of ASV 
radar is to detect: (1) surface vessels and (2) sub- 
surface vessels operating with only a small part such 
as the Schnorkel tube exposed. The secondary func- 
tion is navigation. A system known as the airborne 
early warning [AEW] has been designed and flown 
which is capable of detecting coast line at 200 miles 
and small surface vessels at 120 miles. Thus the sec- 
ondary function and the first half of the primary 
function of ASV radar has been accomplished, in so 
far as the horizon limitation permits, for all but the 
greatest altitudes. It does not seem unreasonable to 
assume that an extension of present radar techniques 
would permit the detection of surface vessels on the 
horizon by a plane flying even at 50,000 ft. 

On the other hand, the best ranges that have been 
obtained on a Schnorkel using the most modern tech- 
niques is less than 20 miles. This is the big problem 
facing the designer of an ASV system today, and it is 
an urgent problem, for recent developments seem to 
emphasize the importance of the undersea vessel as a 
weapon of war. The obvious direction of attack would 
be to design higher-powered radar systems with 
larger antennas and more sensitive receivers incor- 
porating improved anti-sea-return circuits. Common 
sense would indicate, however, that there is a limit as 
to how far it is possible to go in such a direction, for 
the equipment becomes too large and heavy to be 
airborne. Present techniques indicate that this limit 
would be reached before a satisfactory maximum 
range on the Schnorkel was obtained. 

Thus, in closing, it is suggested that although 
radar has succeeded admirably in fulfilling most of 
the requirements of an ASV system, radar may not 
be the answer to the Schnorkel. 






PART II 


RADAR BOMBING 


r 


RESTRICTED 



Chapter 6 


THE BOMBING PROBLEM 


6.1 RADAR BOMBING 

The use of radar for over-land bombing was a 
logical development of its air to surface vessel [ASV] 
application. The following chapters will describe some 
aspects of the bombing phase of military airborne 
radar which greatly increased the effectiveness of air- 
plane warfare during World War II. In particular, an 
attempt has been made to describe the technical de- 
velopment of radar bombing computers as well as the 
requirements that the radar system had to fulfill. 
Wherever possible, the advantages and disadvan- 
tages of various bombing schemes are discussed al- 
though no attempt is made to evaluate critically the 
several bombing methods. 

In this chapter, the geometry of bombing and its 
general nature are described. The treatment of the 
bombing problem is specialized in that the airplane 
is assumed to fly a straight and level course and to 
maintain the same altitude on the bombing approach. 
A more generalized case is discussed in Chapter 9 
although there also the airplane is assumed to be 
flying at constant altitude. A brief description of the 
Norden optical bombsight is given in the present 
chapter since it is intimately connected with the de- 
velopment of radar bombing computers. 

Chapter 7 lists several airborne radar mapping 
systems that are used for bombing and gives the re- 
quirements that bombing places on them. Chapters 
8 and 9 describe several types of radar bombing com- 
puters that are used with airborne radar systems. 

Chapters 10 and 11 describe bombing methods 
that make use of radar installations on the ground as 
well as in the airplane. In Chapter 10, beacon bomb- 
ing schemes such as Oboe and Gee-H are considered 
and bombing by use of radar gunlaying equipment is 
discussed in Chapter 11. 

Toss bombing is discussed in Chapter 12. Although 
this is a method of bombing, it is more directly re- 
lated to fire control (Part IV) than to the other 
bombing schemes considered here. Since the applica- 
tion of radar to toss bombing has been only partially 


exploited, further development of this phase of radar 
bombing is to be expected. 

Finally, in Chapter 13, the assessment and training 
phases of the radar program are considered. As a 
result of the newness of the radar method, no pro- 
vision for an adequate radar training program existed 
at the start of World War II. Although the training 
program for radar operators and mechanics was well 
advanced by the end of hostilities, training was a 
weak link in the radar effort. Chapter 13 recounts 
some of the flaws and omissions that developed in the 
hastily devised radar training program. 

6.2 GENERAL CONSIDERATIONS 

The general problem of bombing from aircraft di- 
vides itself rather naturally into four distinct parts. 
They are: navigation to the target area, identification 
of the target, computation of a release point, and 
steering to that release point. In past bombing com- 
puters, the greatest emphasis has been placed on 
computation and steering to the release point and 
until recentty the computer has not assisted in 
identifying or navigating to the target. This has 
often caused gross bombing errors in combat because 
of the resultant faulty navigation and misidentifica- 
tion. It should be remembered that navigation and 
identification are part of the bombing problem, and 
indeed a very important part. 

6.2.1 Navigation to the Target Area 

Fundamentally, all air navigation is based on dead 
reckoning. If the present position of the aircraft is 
known, then the future position can be determined 
provided the ground velocity vector (consisting of the 
ground speed and direction on the ground in which 
the aircraft is moving) is known. Usually the ground 
velocity vector is obtained from a combination of the 
aircraft vector (consisting of aircraft heading and air- 
speed) with the wind vector (consisting of wind direc- 
tion and speed). Thus the future position of an air- 


RESTRICTED 


37 


38 


THE BOMBING PROBLEM 


craft can be forecast if a point of departure (a fix), the 
aircraft vector and the wind vector are known. 

The problem of finding the aircraft vector appears 
comparatively simple, since every plane carries a 
compass and airspeed meter; but when it is desired 
to perform dead reckoning with an accuracy suffi- 
ciently high for good bombing, this problem becomes 
rather difficult, and special devices must be em- 
ployed. These devices will be discussed in a later sec- 
tion. 

The wind vector may be found by measuring the 
drift of points on the ground relative to the aircraft 
on two different headings. The visual drift meter is 
the most common instrument for determining the 
wind vector in this manner. Another method of find- 
ing the wind vector is to determine two points of de- 
parture, or fixes; if the time lapse between these 
fixes and the aircraft vector while flying between 
them are known, the wind vector may be determined. 
In the following paragraphs, various means of de- 
termining fixes, and thus the wind vector, are dis- 
cussed. 

Pilotage 

Pilotage, though not generally regarded as a form 
of dead reckoning, might be considered as the most 
simplified form of dead reckoning, where the wind is 
more or less guessed at and the points of departure 
are determined merely by looking at the features of 
the terrain (either by radar or visually) . The record- 
ing and combining of the aircraft and wind vectors 
with the point of departure are performed mentally. 

Although pilotage might seem the easiest method 
of navigation, it is such only under ideal conditions. 
Unless the navigator is thoroughly familiar with the 
terrain over which he is flying, any distraction from 
the job of pilotage, such as visual obscuration of the 
ground, violent maneuvering when using an unsta- 
bilized radar system, or the necessity of handling 
guns, may cause him to lose his place and valuable 
fuel may be expended while he finds it again. Some 
terrain (such as the Midlands of England) appears 
uniform for long stretches to all but the most experi- 
enced navigator, thus necessitating some other 
method of navigation than pilotage. 

On the whole, since pilotage is to such a large ex- 
tent an art, and depends entirely on the ability to see 
the ground, and to identify terrain features, it is not 
to be recommended as a means of combat navigation. 
A continuous record of the wind vector and of the 


aircraft position should always be kept, so that, de- 
spite injury to the navigator, damage to the radar 
set, or obscuration of the ground, the pilot will know 
where he was at the time the emergency arose and can 
then choose a course to his home base. Such con- 
tinuous navigation plots were found to be very neces- 
sary and hence were greatly emphasized by the RAF 
during World War II. 

Celestial Navigation 

Celestial navigation is a means of obtaining points 
of departure from sightings on celestial bodies so that 
dead reckoning may be carried out. Although this is a 
reliable and fairly accurate method of navigating an 
airplane, it does have several disadvantages for com- 
bat navigation. 

In the first place, it assumes the absence of an 
overcast. This is often a poor assumption, particu- 
larly under combat conditions when weather fore- 
casting becomes difficult and when missions must be 
undertaken despite forecasts of bad weather. 

Secondly, in order to obtain any reasonable ac- 
curacy in the sightings, and thus in the point of de- 
parture, the plane must provide a stable platform 
for the navigator. This is not always possible under 
combat conditions or in rough air. 

Finally, even if the above conditions are satisfied, 
the accuracy of the point of departure found is not 
high since it is about plus or minus 5 miles. 

The foregoing disadvantages, together with the 
need for freedom from mental stress during the navi- 
gator’s somewhat complicated computations, make 
celestial navigation undesirable for combat missions. 
A permissible exception would be a long over-water 
flight where other means of obtaining points of de- 
parture are nonexistent. 

Instrument Navigation 

The most common type of instrumental aid to 
navigation is the radio compass, by means of which 
one or more lines of position may be found if ground 
radio stations of known positions can be received. The 
radio compass loses its general usefulness under com- 
bat conditions, however, for the aircraft often are 
out of range of friendly transmitting stations and the 
radio system may be jammed easily. A variation on 
the radio compass type of aid is the ground-based 
direction finding station. Nets of such stations with 
central plotting rooms have been used very success- 


EtfeSTItiCTED: 


]££& 


GENERAL CONSIDERATIONS 


39 


fully to give rapid fixes to aircraft within range of the 
stations. The main drawbacks to such chains are the 
rather limited range and the overloading of their 
facilities if many planes want fixes simultaneously. 

One of the more generally satisfactory navigational 
instruments developed during the war is the so-called 
hyperbolic system , which includes GEE and Loran. 
These systems consist of two pairs of synchronized 
stations transmitting pulses at fixed intervals of time. 
The GEE or Loran receiver permits the navigator to 
measure the difference in time between the arrival 
of the pulses from the various stations and thus, with 
a special map on which lines of equal time difference 
are plotted, he can immediately get a good fix. These 
systems suffer somewhat from jamming, but require 
little effort and thought of the navigator (see Chap- 
ter 10). 

Another highly satisfactory method of obtaining 
points of departure, and thereby the wind vector, is 
by the use of radar mapping systems. When land is 
within radar range, these systems offer pilotage de- 
spite darkness or undercast, complete independence 
of ground-based stations, and relative freedom from 
jamming. Radar beacons may be used with such 
mapping systems, permitting the navigator to meas- 
ure the range and direction of beacons of known 
position on the ground. These beacons are relatively 
jam-proof and may be received almost out to the 
horizon. 

Up to the horizon range or slightly less, airplanes 
equipped with a simple radio communication system 
may be accurately positioned by means of ground- 
based radar stations. The rather low traffic capacity 
of this method of navigation as well as the identifica- 
tion problem and ease of detection by the enemy 
limits its usefulness to special missions or to the 
handling of airplanes in distress. 

Choice of Navigational Instruments 

As may be seen from the foregoing, there are many 
means of finding points of departure and the wind 
vector. Because of the several types of missions for 
which airplanes are designed, it is impossible to make 
any generalization as to what equipment should be 
included in all aircraft. Several factors must be kept 
in mind by the designer. He must insist, above all, 
on reliability, and then upon simplifying the task of 
the navigator. He must adjudge weight and size, 
realizing that additional weight may be worth while 
if it means navigational equipment sufficiently ac- 


curate to serve also as a bombsight. In the following 
sections the adaption of various navigational types 
of equipment to bombing will be considered. 

A more complete treatment of air navigation may 
be found in the R. L. Technical Series and else- 
where. 52, 111 


6.2.2 Identification of the Target 

The location of a target can be determined in two 
ways, first, by seeing and recognizing it and second, 
by making a survey to determine its position in re- 
lation to known positions on the earth’s surface. In 
general, the visual bombsights depend entirely upon 
the first method for identification, while the radar 
techniques involve both. 

How well the target can be seen depends upon the 
resolving power of the sighting unit, the contrast be- 
tween the target and its background, and the presence 
of obstructions. The ability to see is further affected 
by the distance from the target and its size. Once it 
has been seen, recognition will depend upon its 
characteristic size, shape, color (if observed visually), 
and surroundings. The ability to recognize targets 
will also depend upon the permitted observation time 
and the relative velocity of aircraft and target. 

The second method is not identification but rather 
eliminates the need for identification. It consists of 
making a survey of present aircraft position and com- 
paring this with the known position of the target, as 
given by a map or other previous survey data. If the 
aircraft is maneuvered to be over the surveyed posi- 
tion of the target, it can be said the target has been 
located by a method of surveying, whether it can be 
“seen” or not. The most familiar bombing methods 
using these principles are the radar beacon systems 
(see Chapter 10). 

The methods which are actually used for location 
of the target include those using only sight, those us- 
ing a combination of sight and survey, and those using 
only survey. The visual bombsights, because of their 
inherently high resolving power, permit a bombardier 
to see the distinguishing characteristics of a target; 
therefore he is able to use vision for identification 
with much greater likelihood of success than the radar 
bombsights. On the other hand, darkness, haze, and 
cloud cover obstruct visual sights so that it is often 
not possible to see the target, much less identify it. 
For this reason and others, it is desirable to use radar 
sights. However, the angular resolving power of a 


RESTRICTED 


40 


THE BOMBING PROBLEM 


radar system is poor with present equipment (see 
Section 7.1.1). This may result in radar pictures of 
the target which are not recognizable, or, at best, are 
only recognizable by an operator who has had con- 
siderable training in radar scope interpretation. One 
technique developed to ease the task of the operator 
is to use well-defined radar reference points as aim- 
ing points, and then, by a method of surveying, to 
identify the target and bomb it whether it is seen or 
not. This is called offset bombing and employs in 
this case both radar sight and surveying. Other 
techniques employ radar beacons for surveying and 
synchronizing a visual bombsight with the target. 
After the initial location of the target by use of the 
beacons, these methods depend upon the inherent 
high resolving power of the visual bombsight to make 
an accurate bomb run. 

The main difference between visual recognition 
and survey types of identification or location is that 
the former depends upon the judgment of an oper- 
ator, whereas the latter is done by a (mechanical or 
electrical) computer. Since the operator is human 
and flying under adverse conditions, it is entirely 
reasonable to expect greater errors in the former than 
in the latter case. This is illustrated by the bombing 
accuracy figures of both visual and radar bombardiers 
in and out of combat. In both cases, bombardiers 
used discrete, well-identified points as targets in 
practice, and complex targets in combat; the result 
was much poorer bombing when it really counted. 
However, bombardiers using surveying computers 
which were automatic, achieved much more nearly 
the same results in combat as in practice. 

In conclusion, it should be stated that the problem 
of target identification is one which until recently re- 
ceived far too little attention. The excellent results 
given by beacon bombing systems tend to show that 
if the problem of identification is removed from the 
control of the operator and put into the bombing 
computer, the overall accuracy will be much greater. 
Since most beacon bombing systems are effective only 
out to the horizon of the ground-based beacon, other 
bombsights are needed which are capable of giving 
good results out to the limit of the range of the air- 
craft. It is apparent, therefore, that the problem of 
identification should, in some manner, be solved for 
the computers which are used for bombing by sight 
(visual or radar), since these computers can be used 
at any range. A step in this direction has been taken 
by the ground position indicator [GPI] computer dis- 
cussed in Chapter 9. 


6.2.3 The Computation of Release 
Points 

The computation of release points can be made in 
many ways. However, regardless of how it is done, 
the same fundamental data are necessary. A release 
point is, by definition, a point in space at which a 
missile can be released so that it will hit the desired 
target. Actually, for each target there are an infinite 
number of possible release points, each dependent 
upon the conditions under which the missile is re- 
leased. In this discussion, the release point will be 
considered for missiles which are released from an 
aircraft with the object of hitting an air, land, or sea 
target; in each case, the missile is carried to the re- 
lease point by the aircraft and is released at the speed 
and with the heading of the aircraft. 

The following list of definitions covers special 
terminology used in further treatment of the bombing 
problem : 

True airspeed (V a ) is the velocity of an aircraft rela- 
tive to the air mass in which it is flying. True airspeed 
is measured in the direction of the aircraft’s heading. 

Wind (W) is the velocity of the air mass relative 
to the earth. Convention assumes the wind to be 
acting in a direction opposite to that indicated by its 
name, i.e., a north wind indicates a movement of the 
air mass from north to south. 

Ground speed (V g ) is the velocity of an aircraft rela- 
tive to the earth. 

Ground track is the direction of the ground speed 
vector, or the path of the aircraft as it passes over 
the earth. 

Drift angle (5) is the angle between aircraft head- 
ing and the ground track. 

Time of fall ( T f ) is the time from release to impact 
of the bomb. It is a function of the altitude and the 
resistance of the air to the bomb’s fall. 

Trail (T R ) is a measure of difference between the 
impact point where a bomb actually hits, and where 
it would have hit if it had fallen without losing hori- 
zontal speed due to air resistance. Trail is measured 
in the direction of the aircraft’s heading and is a 
function of the type of bomb, the initial velocity of 
the bomb, the resistance due to air, and the time of 
fall. 

Cross trail (T x ) is the perpendicular distance meas- 
ured from impact point to the ground track. It is 
equal to the sine of the drift angle times the trail. 

In general, to compute a release point, it is neces- 


RESTR1GTE1 


GENERAL CONSIDERATIONS 


41 


sary that the following information be available in 
one form or another: 

1. Velocity of the missile relative to the target at 
release time. 

2. Velocity of the missile relative to the air mass 
at release. 

3. Characteristics of the missile after release. 

4. Altitude of missile at release. 

It is then possible to state that a missile released 
at a particular distance and direction from the target 
would hit the target, providing each of the items 
mentioned is known and the target is assumed to be 
moving with a constant velocity. However, since it 
is difficult to build computers to solve bombing prob- 
lems in a completely general fashion, the behavior 
of the airplane on the bombing run is usually re- 
stricted in some particular manner. Further compli- 
cations exist in the very measurement of input data, 
since mechanisms do not exist which present all the 
data in the exact form desired for computation. Be- 
cause of these complications of computing release 
points, most of the present-day bombsights have re- 
quired that the airplane be flying straight and level 
at release point. Under these conditions the geometry 
used for the determination of release point is that 
shown in the vector diagram of Figure 1. 



This geometry makes possible a rigorous solution 
for the bombing problem regardless of altitude. The 
altitude factor, of course, enters into the magnitude 
of time of fall and trail. The various existing com- 
puters use some form of this solution, although, in 
some cases, approximations are made. The most 
common approximation is to assume trail to act in 
the direction of the ground track, and then make a 
correction for the resulting deflection error, which is 
trail times the sine of the drift angle (cross trail). 
This gives a solution which is correct except for a 


higher order range error sometimes known as the 
range component of cross trail. 

It is possible to compute a release point by making 
assumptions such as to the airspeed, wind, and alti- 
tude that should exist during the bombing raid. The 
airplane is then required to satisfy those conditions 
during the actual bombing run. Under such circum- 
stances a large amount of maneuvering and possibly 
repeated runs will be required to satisfy the condi- 
tions for which the release point was computed. The 
more satisfactory technique is to employ computers 
that solve for the release point corresponding to the 
values of airspeed and wind which exist on the actual 
bombing approach. 

6.2.4 Steering to the Release Point 

Once the point at which bombs should be released 
has been computed, the problem of maneuvering the 
bombing aircraft to that point still remains. To be 
able to do this, it is necessary that the present posi- 
tion and ground speed vectors be known. The factors 
which are most used in determining present position 
are altitude, bearing of target relative to aircraft 
heading, angle of inclination from horizontal to tar- 
get, and range to the target. Generally speaking, the 
visual bombsights make use of altitude and angle 
measurements, while the radar sights rely on slant 
range and bearing angle. 

The geometry used to steer the aircraft to the re- 
lease point, regardless of the mechanism by which it 
is done, is shown in Figure 2. From this drawing it 
can be seen that it is necessary for the computer to 
be able, in some manner, to make the ground track of 
the aircraft (OC) coincide with that of the release 
geometry (OD). One method could be to indicate 



the difference between present and desired heading 
(i 0 ) to the pilot so he can steer the correct heading. 
Another method could be to give the pilot an indica- 
tion as to when the ground track of the aircraft 
passes through a point (E) displaced from the target 
by the cross trail (T x ) . Assuming now that the steer- 


RESTRICTED/ 


42 


THE BOMBING PROBLEM 


ing is correct, it is necessary to indicate when the air- 
craft reaches the release point. This can be done in 
several ways, one of wdiich is to compare the present 
and the release ground range (or slant range) to the 
target; when the present range equals the desired 
value, the release occurs. Another would be to com- 
pare the present and desired sighting angle to the 
target. Sighting angle is the angle between a line from 
aircraft to target and the vertical. Once again when 
the instantaneous value becomes equal to the desired 
value, the bombs will be dropped. 

Since there are so very many different ways in 
which the bombing geometry ca|n be solved, it is im- 
possible to consider each one here. However, in the 
next two chapters, the geometry will be discussed for 
several radar bombing computers. 

6.3 SOLUTIONS OF THE BOMBING 
PROBLEM 

6.3.1 The Visual Method 

The Norden Bombsight 

In the previous sections the general level bombing 
problem was outlined. Evidently, many solutions are 


possible by either visual or radar means. Before con- 
sidering in detail the several radar solutions that 
were developed during World War II, it seems ad- 
visable to give a short description of the Norden 
stabilized synchronous bombsight, since it was the 
visual bombsight most universally used by the U.S. 
Army and Navy and has influenced to some extent 
the development of radar bombsights. 

The Norden bombsight may be divided into four 
parts which are the rate end, the telescope and 
mirror, the vertical gyro, and the horizontal gyro or 
stabilizer. 

The Rate End 

The rate end solves the range triangle for the 
tangent of the release angle which is equal to 
| VgT f — T R |(1 /h), where W 0 is the ground speed, 
T f is the time of fall of the bomb, T R is the trail, and 
h is the altitude. The solution is performed by a con- 
stant-speed motor driving a disk upon which bears a 
roller (see Figure 3). The roller, in turn, drives the 
mirror, into which the telescope looks, so as to keep 
the range cross hair in the telescope constantly on the 
target. 

The rate of the constant-speed motor is controlled 
by the disk speed knob on the outside of the rate end, 



SOLUTIONS OF THE BOMBING PROBLEM 


43 



and is equal to K/T f , where K is a constant of design. 
The distance of the roller from the center of the disk 
is controlled by the trail-setting arm and by the rate 
knob. 

The roller speed is constant for a given disk speed 
setting and a given distance between the roller and 
the center of the disk. The roller is geared to the 

A 

A 


t„ 



Figure 5. Geometric function of mirror drive rack. 


mirror drive rack which carries a stud in one end 
which engages a slot in, and thereby drives, the 


mirror drive arm. (See Figure 4.) The mirror drive 
arm, therefore, swings at a varying rotational ve- 
locity, so that, for the correct roller speed, the line 
of sight of the mirror remains pointed at the same 
area of the ground throughout the bombing run. 

The rack speed, or roller speed, which is necessary 
to synchronize the mirror on a given spot on the 
ground is | W g k \/h, as may be seen from the follow- 
ing. Suppose that at time k, the airplane is over point 
Q which is at a distance a from the target, and at time 
t 2 the airplane is directly over the target. (See Fig- 
ure 5.) If the line of sight of the mirror is along the 
line AB and its direction is determined by the triangle 
with the sides k, b, and c, where k is of fixed length, 
and b is the rack length necessary to point the mirror 
at the target, then at time t 2 , it will be seen that the 
rack length must be zero. Also, since there are two 
similar triangles, b/a = k/h, or b = ak/h. Divide 
both sides by (k — k). Since a/(k — k) equals 
| V g |, the ground speed, while b/ (k — k) is the rack 
speed R s , R s = V g k/h. 

As previously stated, the rack speed is determined 
by the disk speed D s and the distance d of the 
roller from the center of the disk. Thus R s is propor- 
tional to D,d = Kd/Tf. Combining the two expres- 
sions we have obtained for the rack speed, it will be 
seen that d is proportional to | V g Tj \/h, for syn- 
chronization on the target. 


44 


THE BOMBING PROBLEM 


The distance d is determined by two factors: the 
setting of the trail arm, and setting of the rate knob. 
When the rate knob is turned, it also moves the rate 
indicator shown in Figure 4 whereas when the trail 
arm is moved, it does not move the rate indicator, 
but only the roller. Suppose now that the rate indi- 
cator and trail arm are both at zero. Then the roller 
will be in the center of the disk and will not turn. 
When a value of trail is set in, the roller will move off 
the center of the disk and start to turn. (The trail 
indicator is graduated in mils so that actually a value 
of d = |T* | /h, rather than d = | T R |, is set in.) 

The rate knob is now adjusted to bring the mirror 
into synchronization with the target, so that the 
total distance of the roller from the center of the disk 
is proportional to | V g T f \/h. However, since only 
part of the total motion of the roller was due to 
the rate setting (the remainder coming from the trail 
setting), the angle marked by the rate indicator will 
be tan -1 | V g T f — T R \/h, which is the desired re- 
lease angle. Automatic release is obtained when the 
sighting-angle indicator coincides with the rate indi- 
cator as shown in Figure 4. 

The Mirror and Telescope 

The mirror and telescope assembly is of impor- 
tance not simply for the purpose of seeing the target, 
but also because it provides a method of setting in 
the cross trail correction. By means of a complicated 
system of levers and cams, the trail angle is multi- 
plied by the sine of the drift angle and the whole 
telescope and mirror assembly is tilted proportion- 
ately, so that the ground track of the plane appears 
to pass through the target, although actually it passes 
to one side by an amount equal to the cross trail. 

The Vertical Gyro 

In order to measure the sighting angle to the tar- 
get accurately, a vertical reference line is necessary. 
Such a vertical is provided by a gyroscope that is 
incorporated in the sight. This gyro is connected to 
the mirror and telescope assembly in such a manner 
that once the gyro has been brought into the vertical, 
it will maintain the main axis of the assembly verti- 
cal, although, as mentioned above, the assembly may 
be tipped from its main axis by an amount propor- 
tional to cross trail. 

The gyro is provided with two precessing knobs 
and two bubble levels so that its axis may be brought 
into the vertical. Perhaps the greatest source of in- 


strumental error in using the Norden sight arises 
from the necessity of leveling this sight gyro just 
before the bombing run is begun. In order to level it 
properly, the plane must be flying perfectly even 
and free of all accelerations, and the bombardier must 
be free to concentrate on his task. It is difficult, 
though not impossible, to obtain all these conditions 
simultaneously when flying in combat, thus partially 
explaining why combat bombing results are seldom 
as good as those obtained while training. 

The Stabilizer 

In order to set up a reference line from which to 
measure drift, and thus solve the azimuth problem, 
the bombsight incorporates a horizontal gyro, which 
is maintained in the horizontal plane by means of a 
servo motor. This stabilizer gyro is connected to the 
bombsight through a mechanical system, including 
a clutch, and thus fixes the line of sight in one direc- 
tion relative to space. A potentiometer is incor- 
porated in the stabilizer, with the winding attached 
to the stabilizer case which is fixed relative to the 
aircraft, while the potentiometer arm is attached to 
the gyro through a clutch. Then, by using the elec- 
trical information supplied by the potentiometer the 
aircraft may be returned to its original direction 
either by the pilot using a pilot's direction indicator 
[PD I], or by the automatic flight control equipment 
whenever the aircraft turns away from the direction 
indicated by the gyro. 

There are two knobs on the bombsight for solving 
the azimuth problem. The drift knob turns the air- 
craft without turning the bombsight relative to 
space, thus setting in a drift angle between the line of 
sight of the bombsight and the aircraft, and the turn 
knob turns both the aircraft and the bombsight. The 
two knobs are arranged so that they may be turned 
together (double gripped) and have a double grip- 
ping ratio of 5.25 to 1 (the plane turning more than 
the drift angle is increased). 

The Norden sight is an extremely accurate mecha- 
nism permitting remarkably precise bombing. This 
remarkable precision, however, is obtainable only un- 
der special circumstances, such as perfect visibility, 
calm air, a stable aircraft, and absence of nervous 
tension on the part of the operator — conditions 
which are seldom met in combat. Thus it is possible 
that a bombsight not inherently so accurate as the 
Norden might achieve the same combat record. 
This last fact should be borne in mind when develop- 
ing any new bombsight, since the basic problem is 


ESTRICTED 


SOLUTIONS OF THE BOMBING PROBLEM 


45 


not mechanical accuracy, but rather accuracy under 
combat conditions. 

6.3.2 The Role of Radar in 
Bombing 

In general, the accuracy of bombing can be re- 
garded as an inverse function of the work required 
of the operator, and a direct function of the inherent 
accuracy of the computer. Since the efforts of a 
human operator are affected by the confusion, com- 
fort, physical labor, and mental stress under which 
he operates, many factors should be considered when 
making assessment of a bombing system. Some of 
these factors are : 

1 . Facility with which the computer may be used . 

a. Amount of thinking required of the operator. 

b. Extent to which computer operates auto- 
matically. 

c. Extent to which the computer aids in navi- 
gation and identification. 

d. Training necessary to use computer satis- 
factorily. 

2. Relationship between time needed for setting 
data into computer and time available for com- 
putations and release. (As aircraft with higher 
speeds are developed, the latter time will be- 
come shorter.) 

3. Inherent accuracy of computer. 

4. Requirements which computer places on auxil- 
iary equipment (such as compass). 

5. Types and visibility of targets to be bombed. 

In visual bombing, the operator must make an ob- 
servation to determine the identity of the target, 
and then go through a routine procedure of knob 
turning to drop the bombs. The bombing computer, 
i.e., the bombsight, establishes the release point and 
steers the aircraft, but does not aid in navigation. 
The knob-turning procedure is one which, with suffi- 
cient training, can become automatic for the bom- 
bardier. The visual sighting procedure requires only 
training in identification from the air, since the 
targets are being observed with the eyes that the 
operator has been using all his life. 

The mechanism by which one visual bombsight 
operates has been given in the previous section. This 
method is characteristic of all visual sights in that it 
depends upon the high resolving power of an optical 
system to give an accurate measurement of dropping 
angle. The inherent accuracy of such computers as 


the Norden sight can be high, as has often been 
shown in practice. This particular computer requires 
very little maintenance. Moreover the size and weight 
of the optical computer is not excessive. 

As aircraft with higher speeds are used for bomb- 
ing, the problem of making visual bomb runs will be- 
come increasingly difficult. The reason for this is the 
limit in range at which targets can be seen visually 
even under the best conditions, so that the use of a 
high-speed airplane reduces the time available for 
bombing computations. Offset bombing techniques 
and radar-visual combinations have been suggested 
to help this situation. A visual ground position indi- 
cator has also been proposed as an answer to the 
limited range problems of visual bombsights. Such a 
system would also help in the identification of com- 
plex visual targets. 

Another shortcoming of visual bombing is the in- 
ability to see targets through cloud cover and dark- 
ness, or smoke screens. This is the greatest drawback 
to visual bombing and is the biggest argument for 
radar. 

With the radar bombsight many varied solutions 
can be made. Usually these depend upon the in- 
herently high range accuracy of radar systems. The 
facility with which the several computers can be 
used varies greatly; however, one problem, common 
to all radar bombsights with the exception of beacon 
systems, is that of oscilloscope interpretation. Seeing 
targets on a scope is something new to operators and 
a certain degree of skill is needed in order to recog- 
nize objects. Thus the training required to permit 
intelligent operation of a radar system is definitely 
greater than for a visual system. The knob-twisting 
procedures of radar bombsights, however, can be 
made just as routine and simple as for their visual 
counterparts. 

The radar systems have an advantage over visual 
systems when used in high-speed aircraft since they 
can see farther and thus permit more time for ad- 
justments on the bombing run. Also, of course, radar 
can be used through clouds, darkness, and smoke. 

The inherent accuracy of some radar bombing sys- 
tems, e.g. Shoran (Chapter 10), can be made as high 
as that of a visual system. In general, however, radar 
systems are more complex and require more main- 
tenance. 

The following chapters give detailed discussions of 
some types of radar bombsights. They have been di- 
vided into computers for airborne radar mapping 
systems, beacon bombing systems, ground-controlled 


46 


THE BOMBING PROBLEM 


bombing systems, and toss-bombing computers. The 
effective area coverage of the radar mapping bombing 
systems is far greater than that of the other systems, 
but mapping systems still place the responsibility for 
navigation and identification on the operator. Since 


the bombardier does not have this responsibility 
when beacon and ground-control systems are used, 
these have proved more effective than radar mapping 
systems over the limited areas in which it is possible 
for them to operate. 




Chapter 7 

AIRBORNE RADAR SYSTEMS FOR BOMBING 


7.1 INTRODUCTION 

Although there are many ways in which radar sys- 
tems can be used for bombing, the greatest part of the 
radar bombing done in World War II was accom- 
plished by the use of radar mapping systems. As with 
the optical bombsight, such radar bombing systems 
are independent of ground installations and restrict 
the operating range of the bombing airplane only by 
reducing the gasoline load that may be carried. 

The fundamental components of an airborne radar 
bombing system are: (1) a good navigational radar 
set, with sufficiently high resolution, (2) a method for 
accurately measuring distance by radar, and finally 
(3) a bombing computer that will enable the radar set 
to drop bombs at the correct release point. It is ap- 
propriate that each of these components should be 
considered in a discussion of airborne radar bombing 
systems. However, the theory and design of naviga- 
tional radar systems 51 • 52 and of radar ranging cir- 
cuits 48 have been reported in detail in other publica- 
tions. In view of this, only the requirements that 
bombing places on radar systems will be discussed in 
this chapter. In particular the shortcomings of radar 
search systems, the principles of radar ranging, and 
the additional maintenance problems will be briefly 
considered 

Inasmuch as a comprehensive treatment of bomb- 
ing computers for radar mapping systems is not yet 
available, an explanation of a number of these com- 
puters will be undertaken. In Chapter 8, bombing 
computers that are designed primarily for establishing 
the correct bomb-release points will be considered. 
Finally, a particular type of computer, ground posi- 
tion indicator [GPI], that also provides navigational 
information will be discussed in Chapter 9. 

7.2 AIRBORNE RADAR MAPPING 

SYSTEMS 

7.2.1 Fidelity of Radar Mapping 

In Chapter 6 the bombing problem was divided 
into four parts, the first two of which were navigation 
to the target area and identification of the target. 
Since these functions of the bombing system are very 
important and since they are generally performed 
from the information provided by the radar mapping 


system, the necessity for well-designed navigational 
radars is obvious. 

Ideally, the plan position indicator [PP1] of the 
radar system should portray as a miniature map the 
area being scanned by the radar beam. Thus, 10,000 
square miles or more of the terrain beneath the air- 
craft is reproduced on the screen of the radar PPI. 
The maximum area that can be mapped is deter- 
mined by the range of the radar system employed and 
is of considerable interest for navigation over strange 
country as well as for bombing with high-speed air- 
planes. A short discussion of the factors affecting 
range will be given in Section 7.2.2. First, however, 
the range is assumed to be adequate and the various 
factors that affect the fidelity with which the features 
of the terrain are reproduced on the radar map are 
analyzed. 

Some of the distortions that are commonly present 
in radar maps are : 

1. Slant-range, ground-range distortion, 

2. PPI spot size distortion, 

3. Inadequate azimuth resolution, 

4. Inadequate range resolution, 

5. Distortion caused by unsatisfactory antenna 
patterns, 

6. Distortions arising from motion of the airplane, 

7. Distortions from system errors in range and 
azimuth. 

Each of these factors affecting the fidelity of the 
radar map will be discussed briefly here, but the 
reader is referred to the R. L. Technical Series for a 
more complete analysis. 51, 52 

Slant-Range, Ground-Range Distortion 

Since airborne radar systems measure directly the 
distance from the transmitting antenna to reflecting 
objects on the ground (slant range) the simplest pre- 



RESTRICTEeSF 


47 


48 


AIRBORNE RADAR SYSTEMS FOR BOMBING 



Figure 2A. Radar mapping. Radar view of Boston, Massachusetts taken at an altitude of 4,000 ft (radar wavelength 
1.3 cm). Range: radius 5 nautical miles ground range. Location of aircraft: 42° 21' N; 71° 03' W. 


sentation on a PPI depicts the slant range to the 
target (see Figure 1). At low altitudes, ground range, 
which is displayed on an ordinary map, is very 
nearly the same as the slant range and the radar map 
looks very much like an ordinary map. (See Figures 
2 A, 2B.) 35 However, as the altitude is increased the 
slant range differs more drastically from ground range 
and the radar map is distorted accordingly. It is at 
once evident that objects almost beneath the aircraft 
will be foreshortened more than those at a consider- 
able distance. This distortion has a direct bearing on 
radar bombing because at the time that the bombs 
are released the ground range rarely exceeds one-half 
of the slant range to the target. 

Another consequence of presenting a slant-range 
radar map is the absence of signals for slant ranges 
that are less than the altitude at which the plane is 
flying. The result is a blank circle in the center of the 
PPI presentation (see Figure 3). Although this dis- 


tortion is not troublesome for objects at a consider- 
able distance from the airplane, it greatly affects the 
radar presentation of a target as the bomb release 
point is approached. It is common practice to remove 
this blank circle in the PPI display by synchronizing 
the start of the radial time base with the return of the 
first echo, i.e., the echo from the land or water 
directly beneath the aircraft, the so-called altitude 
signal. This also introduces distortion but of a differ- 
ent character. 52 

The ideal display would present a ground-range 
radar map on the PPI by adjustment of the sweep 
circuits of the PPI tube. For the details of one suit- 
able design, consult the bibliography. 51 Three ad- 
vantages to bombing from such a display are (1) the 
radar maps would correspond more closely to the 
ordinary maps and make identification simpler, 
(2) all ground objects would move with a constant 
velocity with respect to the aircraft, and (3) the shape 


■ ITUCTED 


AIRBORNE RADAR MAPPING SYSTEMS 


49 



Figure 2B. Radar mapping. Ground map of area shown in radar view. 


of large ground features on the radar map would not 
change with range. The second advantage may re- 
quire a few words of explanation. Where a slant- 
range presentation is used, objects on the ground 
toward which the airplane is moving appear to slow 
down as they are approached, even though the air- 
plane is traveling with constant speed. At large dis- 
tances from the aircraft, a change in ground range 
corresponds to a nearly equal change in slant range, 
whereas, near the airplane, a large change in ground 
range will correspond to only a small change in slant 
range, so that for constant ground velocity a reduc- 
tion in slant-range velocity will be observed. The 
apparent change in velocity of the target may be dis- 
turbing to a radar bombardier who is attempting to 
track the target on the PPI display. 

PPI Spot-Size Distortion 
Just as the size of his brush determines the fine- 
ness of the line that an artist may paint, so also is the 
detail of a radar map dependent upon the spot size of 
the PPI. If 2.25 in. on the PPI corresponds to 22.5 


nautical miles on the ground then the radar map re- 
duction is approximately 730,000 to 1. Therefore, if 
the smallest bright spot on the PPI (the spot size) is 
0.015 in. in diameter, then the spot size will corre- 
spond to 730,000 times 0.015 in. or nearly 1,000 ft in 
slant range. Under these conditions it would be very 
difficult to see a river less than 1,000 ft wide on such 
a radar map. Of course, if the area of the radar map 
should correspond to less area on the ground, the in- 
fluence of spot size would be proportionally reduced. 
The minimum spot size of the PPI also affects the 
ability of the radar observer to distinguish between 
objects at about the same range that are slightly 
different in azimuth. 

The spot size distortion may be ameliorated by de- 
creasing the reduction ratio of the radar map through 
the use of larger PPI oscilloscopes having the same 
spot size (if such become available, whereas in pres- 
ent designs the spot has a diameter approximately 
3^250 of that of the tube) or by the displacement of the 
center of the presentation to the side of the PPI so 
as to increase the map area for the region of interest. 


RESTRICTED y 


50 


AIRBORNE RADAR SYSTEMS FOR BOMBING 


The development of new PPI oscilloscopes with even 
smaller minimum spot sizes would also be desirable 
although probably more difficult. 

Another aspect of the spot-size problem is its de- 
pendence on signal intensity. Thus, a very strong sig- 
nal will take up more area on the radar map than will 
a weak signal. Even if such strong signals are limited 
in amplitude before being applied to the PPI, an 
effective increase in the size of the picture element is 
observed, since the intensity of the electron beam 
hitting the fluorescent screen is at maximum value 
for a longer period of time. The use of special circuits 
such as the three-tone presentation (see Section 
7.2.2) increases the range of signal amplitude that 
may be handled by the PPI and reduces this effect 
on spot size. 

Inadequate Azimuth Resolution 

Resolution of radar systems is a measure of their 
ability to distinguish between small reflecting objects 
that are close to one another. Azimuth resolution is 
the ability of a radar system to distinguish between 
objects at essentially the same range but which 
differ slightly in azimuth. The azimuth resolution is 
directly dependent upon the narrowness of the radar 
beam when viewed in the horizontal plane. The beam 
is too wide to resolve two objects, if it has not stopped 
illuminating one before it starts to illuminate the 
other. This means that the radar system sees a con- 
tinuous signal, such as would be returned from a 
single object, rather than from several separated 
objects. 

The width of the beam is inversely proportional to 
the width of the horizontal aperture of the radiating 
antenna and is directly proportional to the wave- 
length of the emitted radiation. Thus a very wide 
antenna, if used with radiation of short wavelength, 
would have a very narrow beam. Mathematically it 
can be shown that 



where 0 = azimuth beamwidth (the angular separa- 
tion of the two points on either side of 
antenna beam where the power is one- 
half of the maximum power emitted in 
the center of the beam), 
a = constant depending on the type of an- 
tenna and definition of d, 

A = wavelength of the emitted radiation, 
d = horizontal aperture of the antenna. 

The horizontal beamwidths of some airborne radar 
systems are given in Table 1. 

It is apparent from this table that good resolution 
for antennas of moderate size requires the use of 
wavelengths of approximately 1 cm. Unfortunately, 
range and propagation problems arise when such 
wavelengths are used as will be shown below. 

Although a very narrow beam would be more de- 
sirable, the mapping that is performed with a 1.0- 
degree beam is satisfactory for identification of parts 
of cities in overland bombing, particularly when there 
are harbors, lakes, or rivers nearby. See Figure 2A 
for an example of mapping obtained by the use of a 
1 -degree beam. 

On the other hand, almost all the radar bombing 
performed by the USAAF in Europe and Japan em- 
ployed antennas having a nominal 3-degree beam- 
width. (Moreover the apparent beamwidth of the 
antenna pattern projected on the ground was much 
greater than this as the target moved under the air- 
craft.) It may be safely stated that this was not ade- 
quate since it resulted in misidentification of many 
radar targets and was partially responsible for the 
relatively low number of direct hits. The use of such 
a broad beam pattern is quite satisfactory for 
area bombing but is entirely unsatisfactory for bomb- 
ing a particular factory in a built-up area unless 


Table 1. Beamwidth of several typical radar systems. 


System 

Cadillac 

AN/APS-15BM 

or 

APS-35 

AN /APS- 15 

AN/APQ-13 

AN/APQ-7 

AN/APS-33 

AN/APS-34 

K hand 
rapid 
scan 

Wavelength (cm) 

10 

3.2 

3.2 

3.2 3.2 

3.2 

3.2 

1.25 

1.25 

Horizontal aperture of 
antenna (inches) 

96 

96 

29 

29 

60 

190 

29 

29 

29 

Beamwidth (degrees) 

3.5 

0.85 

3 

3 

1.3 

0.4 

3.5 

1.0 

1.0 


AIRBORNE RADAR MAPPING SYSTEMS 


51 



Figure 3. Radar display illustrating the dark central “altitude circle.” 


exceptional care is taken in briefing and in the choice 
of target. On the other hand, the success of AN/ 
APQ-7 (Eagle) bombing in Japan indicates the pre- 
cision that may be obtained with the 0.4-degree beam 
pattern when well-trained crews are used. 83 

Inadequate Range Resolution 

Just as the beamwidth of the antenna determines 
the azimuth resolution of the radar mapping, the 
range resolution is set by the duration of the pulse 
and by the receiver bandwidth, which determines the 
faithfulness with which the radar receiver detects and 
amplifies the echo pulse. When the range resolution is 
just sufficient to permit distinguishing between two 
closely spaced targets, the onset of the echo from the 
more remote of the two must not occur until after 
the end of the signal from the nearer target. Targets 
more closely spaced than these would appear as one, 
since a continuous echo signal would be obtained. 

Specifically, with a pulse of 1 /zsec duration and 
adequate bandwidth in the receiver, the range resolu- 


tion or limit to the separation of distinguishable ob- 
jects in range would be 0.000001 (sec) times 186,000/2 
(miles per sec), which is 0.093 statute mile or about 
490 ft. More generally, of course, this spacing will 
equal the pulse duration times one-half the speed of 
light and hence varies directly with the pulse dura- 
tion. However, if use is made of pulse durations 
shorter than those which can be passed faithfully by 
the receiver, their full potentialities for range resolu- 
tion will not be realized and the resolvable target 
spacing will be more closely proportional to the 
reciprocal of the i-f bandwidth. 

Finally, it might be pointed out that the range 
resolution described above refers to slant ranges and 
the range resolution on the ground becomes poorer at 
steep sighting angles because of the larger change in 
ground range which is then necessary to cause a given 
change in slant range. The pulse duration and band- 
width are, moreover, important in their effect on the 
range performance of a radar system and so require 
that a compromise be made in choosing values for 


iTks'i-hkti:!) 


52 


AIRBORNE RADAR SYSTEMS FOR BOMBING 



Figure 4A. Radar presentation showing the “ring distortion” caused by a faulty vertical antenna pattern. 


these parameters in the design of a complete system. 
As a result, the minimum pulse duration provided in 
many production systems has been about 0.25 /xsec. 
In the usual case, it is reasonable to require that the 
range resolution be at least as good as the azimuth 
resolution for the ranges in which the observer is 
particularly interested. 

Distortion Caused by Unsatisfactory Antenna 
Patterns 

As already pointed out, the horizontal radiation 
pattern of the radar antenna is a factor in the fineness 
of detail that may be presented on the radar map. 
The vertical pattern, however, determines the in- 
tensity of signals at various ranges. In order that the 
ground be simultaneously illuminated beneath and at 
a distance from the airplane, the vertical pattern of 
the antenna is made very broad — usually by re- 


ducing the vertical aperture of the antenna. Further- 
more, it is desirable to have the echoes of similar 
ground objects equal in intensity regardless of their 
range from the aircraft, which makes a special vertical 
pattern necessary. In most operational radar bomb- 
ing equipments used during World War II, a cose- 
cant-squared vertical antenna pattern was em- 
ployed. 50a - 51a For ground painting rather than the 
detection of discrete targets a further modification 
of the antenna pattern will be required. 25 

Departures from the ideal vertical pattern may re- 
sult in rings of maximum and minimum intensity on 
the radar map, particularly at short ranges. Such 
rings in the pattern are objectionable in following a 
target on the PPI during a bombing run, since it is 
quite easy to lose the identity of a particular target 
as it passes through these maxima and minima. This 
ring type of distortion is shown in Figure 4A. 40 Figure 


RESTPICTEI 



AIRBORNE RADAR MAPPING SYSTEMS 


53 



Figure 4B. Radar presentation similar to Figure 4A with improved antenna. 


4B shows a similar picture with this type of distortion 
removed by an improved antenna design. 40 

The two pictures in Figure 4 illustrate still another 
distortion that can be introduced by the vertical an- 
tenna pattern. It will be noted that in Figure 4A the 
signal from terrain almost below the airplane is more 
fuzzy in appearance than similar signals in Figure 
4B. This occurs wherever the beamwidth grows 
wider at steeper angles. However, modern design per- 
mits the constructions of antennas which have a 
beamwidth independent of angle of depression (see 
Figure 4B). 

Distortion Arising from the Motion of the 
Airplane 

The radar map is not an instantaneous radar pic- 
ture of the terrain below the airplane but rather de- 
pends on the persistence of the echoes on the PPI to 


present a continuous picture. Frequently 3 sec or 
more are required to complete one scan of the radar 
antenna, i.e., through 360 degrees, and during this 
time an aircraft, traveling with a ground speed of 
180 mph, will move 0.15 mile. The resulting spiral dis- 
tortion of the radar map is difficult to analyze and 
sets a limit on the accuracy with which the position 
of the airplane may be established from the radar 
map. Remedies for this distortion are the use of 
sector scanning and increased scanning speed. A 
recent laboratory development (rapid scan radar) 
completely overcomes the spiral distortion caused by 
the airplane motion and effectively gives a radar 
motion picture of the terrain beneath the aircraft. 
The rapid scan radar uses a short persistence PPI 
and a scanning rate of 720 scans per min. Another 
advantage of this type of instantaneous display is a 
feeling of reality of the motion of the aircraft over the 


RESTRICTED '] 


54 


AIRBORNE RADAR SYSTEMS FOR BOMBING 


earth that is conveyed to the radar observer. It is 
similar to looking out the nose of the airplane at the 
terrain below. 34 

Aside from its translational motion, an airplane 
may also pitch and roll. In such cases the antenna 
may be pointed down at the ground in one part of its 
scan and up into the sky at another. This means 
that the range displayed in one portion of the radar 
map will be limited. This washing out of radar 
signals on a turn is very objectionable, since the turn 
at the initial point [IP] of a bombing run is always a 
turn toward the target. Therefore, the antenna looks 
toward the ground in the target direction and the 
radar operator must wait until the turn is completed 
before he can relocate the target. Moreover, any 
sharp turns to avoid antiaircraft fire will also cause 
the operator to lose the target. 

The solution for these difficulties is the stabilization 
of the antenna system so that it maintains the same 
orientation with respect to the ground irrespective of 
the movement of the airplane. The reader is referred 
to the bibliography 54 for a discussion of the relative 
merits of the various ways in which this can be done. 
(See also Section 4.1.3.) The importance of some form 
of antenna stabilization for radar mapping systems 
used for bombing cannot be stressed too strongly. A 
further complication is that the azimuth data ob- 
tained from the scanner may not be correct during a 
roll (or bank) of the aircraft. It is therefore desirable 
that corrections be made to the azimuth information 
transmitted to the PPI. 

Distortion from System Errors 

In addition to the inherent types of distortion just 
described there are many types of errors that can be 
introduced by imperfect radar systems. Some of these 
are nonlinear PPI sweep speeds, imperfect PPI dis- 
play stabilization, and irregularities in the start of the 
PPI sweep trace. The most important of these is 
imperfect display stabilization. The weak point of all 
radar mapping systems is the fidelity of reproducing 
the azimuth of the reflecting objects. Although the 
range can be measured with high accuracy, the 
angular position in the radar map may differ from the 
actual bearing by as much as 3 degrees in a common 
radar mapping system (AN/APS-15). 

In general, the radar display is coupled to the 
compass in the aircraft so that the top of the radar 
map corresponds to North. Therefore, for true 
angular reproduction, both the antenna-PPI and the 
compass-PPI servomechanisms must be exact fol- 


low-up systems. Considerable attention should be de- 
voted to this part of the navigational or bombing 
radar design, since the position of an airplane is most 
easily established by a measurement of the range 
and the bearing of an identified landmark on the 
radar display. 

Conclusion 

Several of the more common types of distortion of 
radar maps have been discussed which have a direct 
bearing on the bombing problem, particularly where 
identification of targets is concerned. Since the diffi- 
culty of radar target identification has borne the 
brunt of criticism for the general inaccuracy of radar 
bombing, radar map distortion should be minimized 
in the design of any new radar bombing system. 

7.2.2 Other Design Considerations 
of Radar Bombing Systems 

The two most important requirements for radar 
mapping systems to be used for bombing are an 
accurate radar map of the terrain beneath the air- 
craft and a means for accurately determining the 
range of the target. Other design considerations may 
be divided into three groups, namely, factors affect- 
ing (1) radar range, (2) radar displays, and (3) con- 
venience of operation. 

Factors Affecting Radar Range 

Microwave radar systems are, in principle, limited 
in range by the horizon, no matter how powerful 
their transmitter or how sensitive their receiver. The 
horizon range in statute miles is approximately equal 
to the square root of twice the altitude in feet of the 
radar set, assuming that the reflecting objects are at 
sea level. Thus for an altitude of 20,000 ft, the 
horizon distance is 200 statute miles. 

However, because of the rigid restrictions on 
weight and input power of airborne installations, 
most airborne radar systems are limited in range not 
by horizon distances, but rather by insufficient power 
output and antennas of reduced size. Under these cir- 
cumstances if we assume that the scanner is pointed 
continuously at the target and that there is no at- 
mospheric absorption or refraction, the maximum 
range ( R m&x ) is given by : 51b 



where P = peak power of transmitted microwave 
pulse, 


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AIRBORNE RADAR MAPPING SYSTEMS 


55 


Pm in = power of smallest signal that can be de- 
tected by the radar receiver, 

G = antenna gain along center of radar beam 
relative to an isotropic radiator, 

X = wavelength of transmitted radiation, 

<j = effective cross section of the reflecting 
object. 

It would seem from equation (1) that greater range 
performance could be obtained by increasing the 
wavelength (X). This is not true, however, because 
the antenna gain G decreases with increasing wave- 
length X. Thus for a paraboloid of revolution with a 
dipole at its focus, which was a very common type 
of radar antenna, G is equal to 2.4 (7rA/X 2 ), where 
A is the area of the paraboloid aperture (see also 
Section 15.1.3). For this value of G, equation (1) 
becomes 

= ( 0.36— /jY (2) 

' Pm in 47rX“/ 

The above analysis applies in particular to a search- 
light type of beam rather than a cosecant-squared 
beam that provides uniform echo signals irrespective 
of range out to the maximum range .R max . In the lat- 
ter case, which was mentioned under “ Distortion 
Caused by Unsatisfactory Antenna Patterns / ’ Sec- 
tion 7.2.1, the role of the vertical height of the an- 
tenna in determining maximum range is compara- 
tively minor; the vertical aperture is adjusted to 
reduce the difference between the actual and the 
cosecant-squared pattern rather than to concentrate 
the energy into a more and more narrow beam. As a 
consequence, although equation (1) remains true if 
G represents the gain at the nose of the beam, the 
relation between G, X, and the dimensions of the 
antenna must be modified. 51b 

For a cosecant-squared beam antenna having an 
effective horizontal width d, the maximum gain G is 
equal to (47r/sin 0 O cos 0 O ) (d/X) . Here, 0 O is the angle 
of depression of the nose of the beam and is given, in 
terms of the altitude h and the slant range R of the 
beam nose, by the relation sin0 o = h/Ro. The maxi- 
mum gain of this antenna is equivalent to that of a 
uniformly illuminated aperture of length d and height 
X/sin do cos 6q. As a result of these relationships the 
radar equation (1) becomes: 51b 

hR, cos 6 0 = (— -f- d 2 V- (3) 

VPmin 47T / 

Thus, the inverse relationship between maximum 
range and altitude when working with a cosecant- 


squared beam is evident, as is the fact that the choice 
of wavelength will have only indirect effects upon 
the range performance through the variation with 
wavelength of such factors as a and p min . (For ground 
painting and extended complex targets, it is reason- 
able to suppose that a would vary directly with the 
width of the radar beam.) It is also interesting to 
note that, for constant altitude, the maximum range 
for which such coverage can be provided varies as 
the square root rather than as the fourth root of the 
transmitter power, so long as the antenna keeps the 
cosecant-squared pattern. 

In the following paragraphs the variation of maxi- 
mum range with several factors will be considered: 

Wavelength. As seen in Section 7.2.1 under “ Inade- 
quate Azimuth Resolution,” a decrease in wave- 
length increases the azimuth resolution of the radar 
set, so it would seem advisable to use the shortest 
possible wavelength of radiation. Unfortunately, most 
radiation with wavelengths less than 1.5 cm is highly 
attenuated by rain, water vapor, and molecular 
absorption 51 in the lower levels of the atmosphere. 
Moreover, the generation of large amounts of micro- 
wave power at these frequencies and the design of 
satisfactory circuit components introduces additional 
difficulties. 

In such circumstances, a compromise must be made 
between increased resolution and sufficient range. 
With the advent of high-speed bombers having 
velocities such as 600 mph, the maximum range at 
which a target can be identified becomes very im- 
portant if any corrections are to be made in the 
course of the airplane on the bombing run. On the 
other hand, the use of improved navigational devices 
may make some sacrifice of maximum range ac- 
ceptable in exchange for a marked improvement in 
resolution. Because of this, future radar design may 
use wavelengths of 8 or 9 mm which occur in the dip 
in the absorption curve between the water vapor ab- 
sorption peak near 1.3 cm and that for oxygen at 
0.5 cm. 5lb (For short-range communication systems, 
on the other hand, strong exponential absorption 
might be desirable for security and freedom from 
interference.) 

Output Power. Increasing the output power or in- 
creasing the receiver sensitivity are the other obvious 
methods of extending the usable radar range, since 
the maximum area of the antenna is fixed by aero- 
dynamic considerations. Here, however, to double 
the maximum range, the output power must be 
multiplied by at least 4 for a shaped beam, by 16 for 


56 


AIRBORNE RADAR SYSTEMS FOR BOMBING 


a searchlight-type scanner, and by an even greater 
factor if appreciable absorption is present. The ef- 
fect is, of course, a resultant increase in the size and 
weight of the radar system. 51 Receiver sensitivity 
affects the maximum attainable range in an analogous 
way, i.e., to increase the maximum range, the value 
of p m in would have to be reduced by the same factor 
that P would have to be increased. The maximum 
range is much more dependent on antenna size, but 
increasing this quantity usually introduces practical 
difficulties. 

Scanning. Equation (1) was developed on the 
assumption that the antenna was pointed directly at 
the reflecting target so that the target was continu- 
ously illuminated. If the antenna is scanning, how- 
ever, a reduction in the observed maximum range is 
at once apparent. This is to be expected, since the 
number of pulses of microwave energy that illuminate 
the target will be dependent on the speed with which 
the antenna scans over the target. 

Since the intensity with which the returning echo 
is portrayed on the radar map varies greatly with the 
number of pulses received and since the eye of the 
observer is similarly influenced, a reduction in the 
number of returned pulses has a damaging effect on 
the maximum range at which echoes can be detected. 
For a further analysis of this phenomenon see Chap- 
ter 15 and the R. L. Technical Series. 51 

The reduced echo intensity caused by scanning is 
of particular interest to bombing radar systems be- 
cause the many advantages of a continuous presenta- 
tion of any rapid scan system must be weighed 
against the disadvantage of reduced maximum range. 
Sector scanning is one method of reducing the scan- 
ning loss. It is also obvious that the loss will be 
greater for narrower beams, since fewer pulses per 
scan can be observed, so the loss must be balanced 
against the increased antenna gain G that results 
from the use of a narrower beam. 

Receiver Noise Figure. As already noted, improved 
performance of radar sets can be obtained by in- 
creasing the sensitivity of the radar receiver. How- 
ever, p min has a minimum value below which it is 
theoretically impossible to go. This minimum value 
is the noise signal caused by thermal agitation of 
electrons in the input circuits of the radar receiver. 51 
In actual systems, the amount of receiver noise pres- 
ent is still greater than this theoretical lower limit. 
Thus the overall noise figure, which is the ratio of the 
actual noise to the theoretical noise, was about 50 
(17 db) for radar receivers of 3.2 cm wavelength radi- 


ation in 1942, and had been reduced to 20 (13 db) for 
systems in service in 1945. A further reduction in 
noise figure to 10 (10 db) is possible with 1945 labora- 
tory techniques. This means that the sensitivity of 
receivers for wavelengths in the 3.2-cm region can be 
increased at most by a factor of 10, with a resulting 
increase in maximum range of no more than 2 or 3. 
Increasing the receiver sensitivity is perhaps the most 
desirable method of getting increased range for air- 
borne radar systems, since only a slight burden is put 
upon the installation weight, size, and power require- 
ments of the equipment. On the other hand, in- 
creased sensitivity will make the radar, when used 
for bombing, more vulnerable to radar countermeas- 
ures. 

Receiver Bandwidth. The receiver noise caused by 
thermal agitation of electrons is directly proportional 
to the receiver i-f bandwidth, so that a reduction in 
the i-f bandwidth will result in an increase of receiver 
sensitivity. However, the pulse transmitted and re- 
ceived by the radar system is composed of a spectrum 
of frequencies which must all pass through the i-f 
amplifier, if the receiver output pulse is to be a sharp, 
distinct replica of the input pulse. If this condition 
does not hold because the i-f bandwidth is too narrow, 
some of the higher frequency components of the pulse 
will be missing and a poorly defined pulse will result, 
with a corresponding reduction in range resolu- 
tion (see “Inadequate Range Resolution,” Section 
7.2.1). 

The engineering compromise which yields the low- 
est values of p min that is consistent with good pulse 
reproduction is the use of an i-f bandwidth equal to 
(1.2/r) megacycles, where r is the pulse duration in 
microseconds. This value is not a critical one, since 
for bandwidths twice or half as great as the optimum 
value the change in receiver sensitivity is only 15 
per cent. 

Pulse Length, Sweep Speed, and Recurrence Fre- 
quency. The receiver sensitivity is also indirectly de- 
pendent on the duration of the microwave pulse (r). 
p m in is inversely proportional to r provided the re- 
ceiver bandwidth is always adjusted to the optimum 
value and if the distance covered by the sweep dur- 
ing the emission of the pulse is kept constant . For the 
sweep speeds commonly employed, an increase of 
sweep speed will affect the minimum detectable sig- 
nal (p m i n ) favorably. Accordingly, for slow sweeps, 
with the sweep speed and recurrence frequency kept 
constant, an increase of the pulse duration by a 
factor a and a corresponding decrease of bandwidth 


■ STRIGTED | 


AIRBORNE RADAR MAPPING SYSTEMS 


57 


can result in dividing p m in by the quantity (a) 3 2 and 
is equivalent to increasing the peak transmitter 
power by this same factor. A decrease of recurrence 
frequency by a factor a, that would be required in 
the above example if the average power is to be kept 
constant, may affect the sensitivity adversely by a 
factor of a~%, so that the net gain in the case cited 
would be roughly a factor a. For this reason provision 
has been made for a 5-/xsec pulse on several recent 
airborne radar systems as an aid in long-distance 
search. However, as such long pulses have very poor 
range resolution (see Section 7.2.1, “Inadequate 
Range Resolution”), shorter pulses must also be 
provided for precision mapping. 

Miscellaneous Factors. As is evident from the above 
discussion, there are many factors which may affect 
the maximum range of a radar system and some, such 
as the characteristics of fluorescent screen materials, 
will not be described here. It should be emphasized 
that the material presented in this section concerning 
the basic design considerations of a radar system is 
by no means complete, and the reader is referred to 
the Radiation Laboratory Technical Series 51 for a 
more complete treatment. 

Radar Displays 

Other Types of Radar Mapping Presentation. Most 
frequently, the PPI type of radar mapping (see Sec- 
tion 7.2.1) is used for radar bombing, but some sys- 
tems, particularly for marine work, have employed 
the type B display and variations of the B display. 
The B display is a square radar map wherein target 
range is plotted vertically as the y coordinate and 
azimuth is plotted horizontally as the x coordinate. 
Ordinarily, less than 180 degrees of azimuth are por- 
trayed in a B type map. 

The type B radar map becomes severely distorted 
as the target moves in toward the airplane, which is 
apparent from the fact that the point under the plane 
is displayed as a line. However, for killing drift on a 
bombing run, the type B display is especially helpful, 
since any deviation from a constant azimuth is very 
noticeable and resulting corrections in the airplane’s 
course will be made earlier and more rapidly. For 
over-land bombing, however, where the identification 
problem is severe, an auxiliary PPI is essential. 

Another type of radar display frequently en- 
countered is the delayed PPI. Although the slant 
range distance displayed on the PPI may be only 15 
miles, the start of the sweep may be delayed in fixed 
steps of 10 miles out to as much as 200 miles. The 


resulting display is severely distorted but the ex- 
panded range scale permits very accurate range 
determination. For this reason, the delayed PPI is 
reserved for use with beacons, where it is important 
to determine the ranges to beacons very accu- 
rately. 

Many other modified radar displays have been de- 
vised for special purposes, but a complete listing 
of these will not be undertaken here. Of special inter- 
est is the scheme now employed in the British H2S 
Mark IVa radar wherein the terrain features appear 
to remain fixed in the display while the origin of the 
sweep, which corresponds to the airplane, moves 
about on the fixed radar ground map. Separate con- 
trols are used to position the area being mapped. 33 

Three-Tone PPI Presentation. As seen in Section 
7.2.1, “PPI Spot-Size Distortion,” the output signals 
from radar receivers are limited to prevent blooming 
of the spot size on the radar PPI. This limiting pro- 
vision has a particularly harmful effect upon the 
region of signal strength that can be portrayed on the 
radar map. Thus if the receiver gain is increased so 
that land-water boundaries are visible, the stronger 
echoes from the built-up areas will blend with those 
from the land. On the other hand, if the strong land 
echoes must be differentiated from the very strong 
echoes from built-up areas, a reduced receiver gain 
must be used in which the land-water contrast is en- 
tirely lacking. This makes it very difficult to locate 
built-up areas such as factories by reference to some 
nearby river or lake. 

Electronic devices known as three-tone circuits 
were devised to overcome this difficulty and it is now 
possible to present built-up areas against a fainter 
background of land in which the water areas appear 
black. Figures 4A and 4B show the improvement in 
identification that is possible by using such a three- 
tone attachment. For details of the method of secur- 
ing this desired effect, the reader is referred to the 
bibliography. 49a>51c 

Factors Affecting Convenience of Operation 

The ideal radar set would have only one control, an 
on-off switch. Unfortunately, radar systems used for 
bombing are not so simple, since the job to be per- 
formed is complicated and flexibility is sometimes de- 
sired. As a consequence, a number of controls must 
be used and the resultant complexity of the radar set 
makes it necessary to train radar operators for a con- 
siderable period of time. A sizable reduction in the 
training program could be obtained, however, if more 


RESTRICTE 


58 


AIRBORNE RADAR SYSTEMS FOR BOMBING 


attention were given to the design of simplified auto- 
matic controls. A few such improvements follow. 

1. Completely automatic frequency control [AFC] 
so that the operator does not have to tune in man- 
ually either the echo or the beacon signal. A com- 
bined mapping and beacon presentation is also pos- 
sible. 

2. Optional fixed range marks, preferably showing 
ground range. 

3. A continuously variable range mark with an 
associated dial for use in reading the precise range to 
identified landmarks. 

4. An electronic cursor for measurement of angles 
to the identified landmarks. As in the case of Nosmo 
(Section 8.3.2), this cursor could be either bright or 
dark. 

5. Ganged controls so that sweep speed, pulse du- 
ration, pulse recurrence frequency, i-f bandwddth, 
and range-marker interval would all be shifted by 
one control. This feature has been incorporated into 
the AN/APS-33 radar system. 

The desirability of a simplified control system is 
readily apparent. It may be hoped that the progress 
of radar will be like that of radio, where a pushbutton 
has replaced the minimum of three dials previously 
required to tune in a radio station. The general radar 
experience of World War II has shown that the in- 
creased performance w^hich a number of controls may 
provide in the hands of experts is far outweighed by 
the maladjustment that a confused operator can 
devise. Thus, it is much more desirable to accept a 
known reduction in performance in order to simplify 
the control process than to gamble on the correct ad- 
justment of a larger number of controls that may 
provide a higher performance. 


7.2.3 Brief Description of Several 
Radar Mapping Systems 

The systems described below are offered as a selec- 
tion from existing equipments illustrating the basic 
design principles of the two preceding sections. They 
include systems with considerable field service as 
well as recently developed laboratory prototypes, so 
that trends can be observed. The listing is in chrono- 
logical order of development and includes only de- 
velopments made in the United States. Attention is 
given in this outline chiefly to mapping functions. See 
Table 2 for a brief listing of some fundamental design 
parameters. 

1. ASB-3. This is an early search and homing 
radar, operating at 50 cm. Because of the relatively 
wide beam, no automatic scanning provision is pro- 
vided, the antenna array being manipulated by hand. 
The indicator is a modified A scope. Land masses to 
70 miles and large ships to 30 miles are standard per- 
formance. 

2. AN/APS-3. This Navy X band radar, which 
has seen abundant field service, is designed for 
use at medium and low altitudes (500 to 10,000 ft). 
It has a sector type of scan with a B scope presenta- 
tion. Ranges of 80 nautical miles on single freighters 
have been commonly attained. 

3. AN/APS-15. This is an X band radar system 
provided with tw r o alternate antennas: one providing 
a cosecant-squared pattern suitable for use at high 
altitudes (10,000 to 36,000 ft); the other antenna, of 
higher maximum gain, provides a cosecant-squared 
pattern suitable for low altitude use (500 to 10,000 
ft). With this equipment, which has seen field service 
in great numbers, particularly with the 8th AAF in 


Table 2. Design characteristics of some existing radar systems 



AN/APS-15 

AN/APQ-7 

AN/APS-34 

AN/APS-33 

Cadillac 

AN/APQ-13 
with large 
antenna 

K band 
rapid 
scan 

X (cm) 

G 

P (kw) 
d (in.) 
r (/nsec) 

v r (pulses per second)* 
Beamwidth (degrees) 

Typical installation 

Scan type (degrees) 

3.22 

800 or 1,200 
40 

29 

1 or 0.5 
650 or 1,010 
3 

Ventral 

360 or 
sector 

3.22 

1,500 

50 

190 

0.75 or 0.4 
400 or 1,600 
0.4 

Special 
housing 
± 30 

1.25 

4,000 

30 

29 

0.5 or 0.25 
800 or 1,600 
1.0 

Ventral 

360 or 
sector 

3.22 

1,350 

100 

29 

0.5, 5.0 
800, 200 
3.0 

Ventral 

360 or 
sector 

10 

1,150 

1,000 

96 

2 

300 

3.5 

Special 

housing 

360 

1 

3.22 

1,130 

35 

60 

0.5, 1.13 
1,350, 624 
1.3 

Ventral 

360 or 
sector 

1.25 

4.000 

24 

29 

0.16 

6.000 

1.0 

Ventral 

360 


quenily for^bef^jn^iUciTogadonf ^ * he Va ‘ UC °‘ r ' MoSt ° f ,he radars here des ' ribad ala ° ha™ 


a 2- nsec pulse and a suitable recurrence fre- 




KESTKICTE 


ACCURATE RADAR RANGE MEASUREMENT 


59 


England, land mapping can be done to 40 nautical 
miles, using the high-altitude antenna, and large 
cities can be seen at 90 miles. 

4. AN/APQ-7 (Eagle). This Army equipment 
which has been in limited field service, is designed as 
a high-resolution radar system. It is intended pri- 
marily for high altitude use (25,000 ft). The antenna 
is a 16-ft linear array of dipoles housed in a special 
wing-like nacelle. Scanning is done by an electrical 
method which does not require the dipole array to be 
rotated. Ranges up to 160 miles have been obtained 
on cities. Although giving very superior resolution, 
the system suffers somewhat from the inconvenience 
of the large antenna array and its inability to scan 
outside a 60-degree sector centered about the forward 
direction. 

5. AN/APS-34. This Navy K band radar, which 
went into production in 1945, is designed for high- 
resolution search from medium altitudes (1,000 to 

10.000 ft). Since the operating wavelength of 1.25 cm 
is virtually that for maximum absorption by water 
vapor, the range of this equipment is limited by the 
resulting attenuation. On a dry day near Boston 
(4-5 gm H 2 0 per m 3 ) there is land mapping to 20 
miles. On a moist day (15 gm H 2 0 per m 3 ) this may 
be reduced to 7 or 8 miles. Boston has been seen 
at 35 nautical miles during dry weather. The reso- 
lution is very good. 

6. AN/APS-33. This Navy X band radar is de- 
signed to supplant the AN/ APS- 15 for medium- and 
low-altitude search functions. The use of a long search 
pulse (5 Msec) with r-f band switching and other im- 
provements permits land mapping to about 100 
nautical miles. Large cities have been seen at the 
horizon (120 miles) at 12,000 ft. 

7. Cadillac. This radar system is essentially in the 
early production stages (1945). It is a Navy S band 
high-power system designed for search and special 
uses at medium to high altitude (10,000 to 30,000 ft). 
Land mapping to 120 miles is obtained and ranges on 
large ships are usually limited by the horizon. Cities 
can be seen at 200 nautical miles. 

8. AN/APQ-13 with Large Antenna. The AN/ 
APQ-13 is an Army equipment similar to the 
AN/APS-15. For high altitude operation in large air- 
craft, this basic equipment was modified by the addi- 
tion of a 60-inch antenna. The aim was to provide a 
high-resolution system with as small a protrusion of 
the antenna housing below the aircraft fuselage as 
possible. Painting of land is obtained to 70 miles at 

20.000 feet and 35 miles at 5,000 feet. The antenna 


protrudes only 7 in. below the keel line of the air- 
craft. 

9. Rapid Scan. The Rapid Scan system is a Kband 
development of the Radiation Laboratory which was 
in the laboratory stage at the cessation of hostilities 
of World War II. It produces a very high-resolution 
display at 720 scans per min on a very short per- 
sistence cathode-ray tube. Although the range is 
limited by water vapor attenuation, it embodies a 
novel idea in design which will be very useful where 
very fast moving aircraft are involved. The reported 
land mapping range is about 9 miles on a dry day 
but will probably be improved. 31 

An examination of Table 2 shows that the trend in 
airborne search radars is toward higher power, nar- 
rower beams, antennas giving more uniform ground 
coverage, longer pulses for long-range search and 
mapping, and shorter pulses where good discrimina- 
tion is required. 


7.3 ACCURATE RADAR RANGE 
MEASUREMENT 

7.3.1 Principles of Airborne Radar 
Ranging 

Basically, all radar range measurements are made 
by measuring the time required for a radar pulse to 
travel from the radar transmitter to a reflecting ob- 
ject and back to the radar receiver. The total distance 
traveled by the radar pulse will be equal to the prod- 
uct of the elapsed time by the speed of light, and the 
range to the reflecting object will be one-half that 
distance, or approximately 490 ft per /xsec. Range 
marks can be produced by applying a signal to the 
indicator at certain predetermined times after each 
pulse is emitted from the radar transmitter. Such 
range marks will appear on a PPI as concentric cir- 
cles. 

The range of any reflecting object may be deter- 
mined by comparing its position on the PPI with the 
position of one or more range marks. For navigational 
purposes, interpolation between fixed range marks 
representing range intervals of 1, 5, or 10 miles is 
usually adequate, but more convenient and more 
accurate methods are usually required for use with 
bombing computers. For the latter purpose, a vari- 
able range mark is usually employed and its ac- 
curacy should be consistent with the resolution of 
the radar presentation, i.e., the error in measuring 


^RESTRICTED# 


60 


AIRBORNE RADAR SYSTEMS FOR BOMBING 



VARIABLE range indicator sweep 

MARK TRIGGER 

Figuke 5. AN/APS-15A raDgo unit block diagram. 


1LL1 11-L.l.Lll I I III I 1 I II I II l I I l II I I II il l 

i i i > • i i 

i • i • ii 

J 1 1 i I I i i 

1 I 



J ' ■ « I 


1 


1 1 I I 11 i 1 1 1 1 III 1 1 1 I I I I I I I 1.1 1 1 I Li-Lll I 

1 i ■ ; i i 1 

i ! i i I i i 

1 1 I ! I I L 

i i i 

i I i 

I I i 




' 1 

1 

1 

1 

-1 1 


1 

1 

1 

l 

1 

i ! 


1 

1 

1 

1 

M- 

I 

1 

1 

1 

1 

i 

1 


• 

• 


J "« — 1 ± 


mill 

i 

i 

i 


ONE-MILE RANGE MARKS 
5: 1 DIVIDER 

FIVE-MILE RANGE MARKS 
2.1 DIVIDER 

TEN-MILE RANGE MARKS 

7:1,13:1 , or 24:i divider 

TRIGGER 

DELAY 

TRIGGER 

SELECTOR GATE 

TRANSMITTER TRIGGER 

SWEEP DELAY 

INDICATOR SWEEP TRIGGER 


1 

J 




J 

l 

i 




STEP DELAY 

(MOVES IN 10-MILE STEPS) 

SELECTOR GATE 

INDICATOR SWEEP TRIGGER 
(STEP DELAY) 

RANGE DELAY 

VARIABLE RANGE MARK 


Figure 6. Timing diagram AN/APS-15A range unit. 


slant range should be appreciably smaller than the 
distance corresponding to the range resolution of the 
radar equipment. 

For a description of circuits for producing fixed and 
variable delays suitable for measuring radar ranges, 


consult the bibliography. 48e - 51 The present discussion 
includes a brief description of the range unit of the 
AN/ APS-15A radar bombing equipment and an ex- 
amination of some of the problems associated with 
its use. 



ACCURATE RADAR RANGE MEASUREMENT 


61 


7 . 3.2 Description of a Typical 
Range Unit (AN/APS-15A) 

The range unit of an airborne radar bombing sys- 
tem usually serves as the timing or synchronizing de- 
vice for the entire system. As such, it may determine 
the pulse recurrence frequency and indicator sweep 
timing, as well as provide the delay circuits for range 
measurements. This is true for the AN/APS-15A 
range unit, as may be seen from the block diagram of 
Figure 5 and the timing diagram of Figure 6. 

A crystal-controlled 80.86-kc oscillator is the basic 
timing circuit from which all the triggers and range 
markers of the AN/APS-15A range unit are de- 
rived. The period of oscillation corresponds to 1 
nautical mile of radar range and is used as a standard 
for the calibration of the variable delay circuits, as 
well as to form the fixed range marks and the syn- 
chronizing pulses for the transmitter and indicator 
units. One-mile range marks are generated by differ- 
entiation of 80.86-kc square waves produced from the 
output of the crystal-controlled oscillator. From 
these, a 5/1 divider produces 5-mile range marks and, 
similarly, 10-mile range marks are created with a 2/1 
divider circuit. An additional divider circuit, whose 
dividing ratio can be adjusted to 7/1, 13/1, or 24/1, 
produces a trigger with resulting recurrence fre- 
quencies of 1,155, 622, or 337 pulses per second, 
which are used with transmitted pulse durations of 
0.5, 1.0, or 2.0 /xsec respectively. The last-mentioned 
trigger activates three other circuits: (1) a fixed-delay 
circuit followed by a gate which selects the next 10- 
mile range mark, which in turn is used to trigger the 
transmitter; (2) a variable-delay circuit which initi- 
ates a trigger for starting the indicator sweeps at 
a time corresponding to from 6 miles before the 
transmitter pulse to 6 miles after the transmitter 
pulse; and (3) a step-delay circuit, variable in 10-mile 
steps, followed by a gate which selects any 10-mile 
range mark before the next transmitter trigger for use 
as an alternate indicator-sweep trigger. The 10-mile 
mark selected by the step-delay circuit is also used 
to trigger the variable range-delay circuit which 
produces the variable range mark used for bombing. 

In summary, the range unit described above pro- 
vides a trigger for the radar transmitter; a trigger for 
the indicator sweeps which may either be timed 
within the range of 6 miles before and 6 miles after 
the transmitter trigger, or delayed in steps of 10 
miles each up to 200 miles after the transmitter 
trigger; crystal-controlled range marks of 1-, 5-, and 


10-mile intervals; and a range mark which is variable 
from 0.5 to 15 miles in range and which may be de- 
layed in steps of 10 miles by the step-delay circuit 
for the indicator sweeps. The crystal-controlled range 
marks may be used to calibrate the variable-range 
mark. 

In addition to the intentional delays introduced by 
the circuits discussed above, circuits in the range 
unit and other components of the radar system intro- 
duce additional unintentional and, in most cases, 
undesirable delays which must be considered when 
calibrating the range unit for bombing purposes. 

7.3.3 Problems of Range Calibration 

As mentioned in Section 7.3.2, several character- 
istics of the AN/APS-15A range unit as well as of 
other units of a complete bombing radar system 
must be considered when calibrating the range cir- 
cuits for bombing purposes. Most of these char- 
acteristics would be encountered in other equipments 
of the same general type and are therefore of general 
interest. 

The problem of matching a range mark with a sig- 
nal is, to a certain extent, an ambiguous one which 
depends upon many factors, including the judgment 
of the operator and the adjustment of the equipment. 
The calibration adjustments of the range delay cir- 
cuit are a slope adjustment and a zero adjustment. The 
variable range mark is permanently connected to the 
A scope, where it may be compared with the crystal- 
controlled 1-mile range marks. Coincidence of the 
variable range mark and a 1-mile mark may, with 
care, be established with a reproducibility of about 
25 ft. 

Making the slope adjustment is quite simple, since 
it is necessary only to select two 1-mile marks be- 
tween 1 and 15 miles in range and adjust for the 
proper difference in range-scale readings when the 
variable range mark is made coincident with the se- 
lected 1-mile marks. The particular 1-mile marks 
chosen for this purpose should be those for which the 
slightly nonlinear characteristic of the range delay 
circuit causes the least maximum deviation over the 
ranges of particular interest to the bombing method 
used. 

The zero adjustment is considerably more compli- 
cated than the slope adjustment, and the remainder 
of this section will be devoted to a discussion of 
factors which influence the setting of the zero control. 

One important consideration is the criterion for 


RESTRICTED 


62 


AIRBORNE RADAR SYSTEMS FOR BOMBING 


matching the range mark to a signal on the PPI. The 
range marks are not instantaneous and therefore 
have a finite width on the PPI ; moreover, their wave 
shape is not rectangular but the sides are sloping, 
making an evaluation of their width difficult. Most 
operators prefer to have the outer edge of the range 
mark just touch the inner edge of the signal when 
making an accurate range measurement. Under ideal 
conditions, this would occur when the half-voltage 
point on the tracking edge of the range mark is coin- 
cident with the half-voltage point on the leading edge 
of the signal pulse. The finite spot size of the PPI 
requires that the range mark and signal be slightly 
more separated than under ideal conditions. The 
magnitude of this effect increases as the brilliance of 
the PPI presentation is increased, and is greater for 
slower PPI sweep speeds, which correspond to longer- 
range sweeps. For the latter reason, the sweep speed 
used during a bomb run should preferably be con- 
stant. If a presentation where the target remains in a 
fixed position on the PPI is desired, this should be 
accomplished by a variable offset-center presentation 
rather than by automatic sweep expansion. A thin, 
sharply defined range mark is highly desirable, but to 
achieve this, the circuit through which the range 
mark passes must have a wide bandwidth. 

The crystal-controlled 1-mile range marks indi- 
cate slant range intervals of 1 mile, but because of 
unintentional circuit delays, they do not represent 
slant ranges which are exactly integral numbers of 
miles, that is, a zero error exists. The 1-mile range 
mark which nominally represents zero range arrives 
at the indicator after a small amount of delay caused 
by isolating and impedance matching circuits and by 
the video amplifier of the receiver. This same 1-mile 
range mark, after some delay in the 5/1 and 2/1 
divider circuits, becomes the 10-mile range mark 
which is selected as the trigger for the transmitter. 
Further delay occurs in isolating and impedance 
matching stages and in the transmitter itself. 

The transmitted pulse is delayed (as are radar re- 
turn signals) in the receiver before it reaches the indi- 
cator. The appearance on the PPI of the leading edge 
of the transmitted pulse may be considered as repre- 
senting a signal of zero range, and the interval be- 
tween the trailing edge of the reference 1-mile range 
mark and the leading edge of the transmitted pulse 
may be considered the zero error of the crystal-con- 
trolled range marks. If the zero delay is known, the 
1-mile range marks can be used to calibrate the 
variable range mark. 


If we denote (1) the delay between the generation 
of the zero range mark and the appearance on the PPI 
of its leading edge by d h (2) the delay between 
the generation of the zero range mark and the ap- 
pearance of the leading edge of the transmitted pulse 
by d 2 , and (3) the width of the range mark by w y 
then the zero delay is (d 2 — di — w). All three of 
these quantities are subject to variation if the 
vacuum tubes or certain other elements of the associ- 
ated circuits are changed, or if the characteristics of 
these elements change with time. The quantity d 2 
varies with the setting of the i-f gain control of the 
receiver and w changes with variations in range 
mark amplitude and PPI bias. 

The most effective means of determining the zero 
error of such a radar ranging system is to calibrate 
the variable range mark by means of the radar return 
from an object at a known distance from the radar 
antenna. If this is done, the same adjustments of PPI 
sweep speed, PPI bias, signal intensity, and range 
mark intensity that would be employed in normal 
operation should be used. Unfortunately, this calibra- 
tion procedure is inconvenient and, in the case of 
H2X operations in Europe, was used only with a 
few “sample” equipments to determine an average 
value of zero error which was then used to calibrate 
the many sets used in bombing missions. 

An alternate method of zero calibration uses a de- 
layed sweep to compare the variable range mark with 
the next transmitted pulse. This may be accom- 
plished by using the step delay to start the PPI 
sweep and the range-delay circuit just 10 miles before 
the next transmitter trigger is selected. With the 
range scale set to 10 miles, the zero adjustment is so 
made that the outer edge of the range mark just 
touches the inner edge of the circle on the PPI caused 
by the transmitted pulse. Three objections to using 
this method to calibrate the AN/APS-15 range unit 
were: (1) the sweep speed must be relatively slow 
(greater than 10 miles in range) in order to display the 
transmitted pulse; (2) the position of the transmitted 
pulse on the PPI may vary as much as 150 ft with 
extreme variation in receiver gain adjustment; and 
(3) the leading edge of the transmitted pulse was 
often obscured by the leakage of trigger pulses into 
the receiver circuits. In spite of these objections, this 
method of calibration is easily performed and is 
often used whenever the resultant errors can be 
neglected. 

From the foregoing discussion, it is clear that very 
accurate range measurements by radar cannot be 


RADAR MAINTENANCE 


63 


made without accurate calibration data. It is also 
noteworthy that the range-calibration methods that 
have been used are neither simple nor precise. 

7.4 RADAR MAINTENANCE 

7 . 4.1 Introduction 

The problem of radar maintenance for an airborne 
radar bombing system, namely, attaining and main- 
taining peak radar performance, is the same as for 
an aircraft to surface vessel [ASV] radar system 
(Section 5.1). The addition of such features as range 
units, computers, special antennas, and provisions 
for beacon bombing, to convert radar search systems 
to radar bombing systems, imposes further require- 
ments on the parameters which have to be measured 
for complete maintenance of a radar bombing system. 
Special items of test equipment and test points for 
connecting the new items of test equipment to the 
radar system are required. Instruction literature for 
maintenance must be appropriately annotated, and 
the maintenance training program must be suitably 
expanded. Also, the type of installation of radar 
bombing systems in various aircraft imposes specific 
requirements on the design of such systems to insure 
complete accessibility of the test panel and other ex- 
ternal test points, with particular reference to those 
essential for checking the range unit, computer, and 
antenna alignment. 

7 . 4.2 Special Test Equipment 

Range Unit Test Set 

Most radar bombing systems have range units for 
providing accurate range marks and for furnishing 
requisite information for the bombing computer. It is 
desirable to know whether the range units are work- 
ing properly, both as separate major assemblies and 
also when connected to the radar system proper. 
Also, it is desirable to know and to be able to check 
the calibration of the range units, unless they are self- 
calibrating. The particular checks and calibrations 
to be made will depend upon the design of the range 
unit used. However, general types of checks and the 
reasons for making them will be indicated in the 
following paragraph. 

As indicated in Section 7.3.3, the range units asso- 
ciated with radar bombing systems provide the tim- 
ing or synchronization for the entire radar system. 
Thus, the trigger for the modulator and for the indi- 


cator sweeps is provided by the range unit. In the 
range unit described in Section 7.3.2, the latter trig- 
ger may occur at any time within 6 miles (75 /usee) 
before and 6 miles after the modulator trigger. The 
range unit itself is crystal-controlled and provides 
accurate 1-, 5-, and 10-mile range marks, and a 
variable range mark which can be varied from to 
15 miles in range (for measuring the range of the 
altitude signal and the slant range of the target), or 
which is delayable in steps of 10 miles. It is necessary 
to ascertain if there is jitter in the 1-mile pip gen- 
erator, the 10-mile pips, the modulator trigger, or 
in the phantastron delay, if this is used, since jitter 
of this kind is reflected in range errors. Furthermore, 
since the selector gate selects each 10-mile mark, it is 
important to know that the center of the gate is at 
the center of the selected 10-mile mark; otherwise 
an error in range is introduced. 

Two general types of test calibrators, both crystal- 
controlled, have been designed for calibrating range 
units of this type. One provides accurately spaced 
positive or negative marker pulses which can be ad- 
justed in phase from zero through 360 degrees with 
respect to the trigger which it provides. The range 
marks have a spacing of 500 yards and an accuracy 
of spacing of 0.1 per cent, with a jitter with respect 
to the synchronization pulse of 0.02 /usee. The trigger 
output pulses are positive or negative, up to 50 volts 
amplitude, of 0.8-/usec duration, and have a recur- 
rence frequency of 400, 800, 1,600, or 2,000 pulses per 
sec. This type of test calibrator was used with such 
equipments as the AN/APA-5 and the AN/APQ-7. 

The other type of calibrator incorporates an 80.86- 
kc crystal-controlled circular sweep (type J) and 
three linear sweeps (type A) of 1, 30, and 350 nautical 
miles. These are precision sweeps. The circular sweep 
can be synchronized with an 80.86-kc sine wave, and 
the linear sweeps can be obtained if the unit is trig- 
gered externally (from other than a sine wave oscil- 
lator), although in this latter case the sweeps are not 
precision sweeps. The sweeps can be delayed from 0.8 
to 50 nautical miles with an error of less than 1 per 
cent in linearity; 10-mile points are marked on the 
dial on the face of the cathode-ray tube indicator 
which is included in the unit. A video amplifier, which 
has a 3-mc bandwidth and a fixed gain of 12, is also 
included. A positive or negative trigger output is 
provided at pulse recurrence frequencies between 
160 and 2,000 c. Correspondingly, the test calibrator 
can be synchronized by positive or negative triggers 
as indicated above, at pulse recurrence frequencies 


64 


AIRBORNE RADAR SYSTEMS FOR BOMBING 


up to 400 c. Units of this kind were used with systems 
such as the AN/APS-15 series and the AN/APA-30. 

Computer Test Set 

The computer is as much an integral part of a 
radar bombing system as the range unit (see Chap- 
ter 8). The maintenance requirement for a computer 
depends entirely upon the nature of the computer 
and its connection to the range unit. Thus, for a 
manually operated computer, such as used in the 
AN/APS-15, the bombing problem is solved by 
manually feeding to the computer information ob- 
tained from the range unit (such as altitude and 
slant range), the airspeed, the drift angle, and informa- 
tion obtained from tables for the type of bomb used. 
A computer of this kind requires no special mainte- 
nance other than that required for normal satis- 
factory mechanical operation; nor does the bombing 
position computer for Shoran (see Chapter 10). The 
Shoran computer is more complicated than the 
manually operated computer mentioned above but 
is designed so that calibration of the range com- 
ponent of trail, the limits of cross trail, the limits of 
range component of trail, and the speed check 
(measuring the time required for the computer to 
run out to 4 miles, on the speed counter) can be 
carried out without special test equipment. 85 In this 
case, it is necessary only to detail the calibration and 
checking procedure. Other computers, such as the 
one designed for the GPI (see Chapter 9), are self- 
calibrating also. 

One type of computer test set has been designed to 
check the ability of a radar system to track smoothly 
and accurately. This test set also checks the hori- 
zontal range mechanisms and measures with great 
accuracy the voltages developed in various portions 
of the computer. A vacuum tube nullmeter compares 
a known adjustable voltage with the voltage being 
measured, and reads zero when they are equal. Range 
is indicated in terms of voltage ratios which are then 
multiplied by the proper scale factor for the equip- 
ment under test to obtain the actual distances. The 
accuracy of the potentiometer setting is one part in 
2,500, and that of the feed setting is one part in 1,000. 
Manual or motor operation is provided. 

Another type of computer test set, such as is used 
in toss bombing (see Chapter 12), provides a doppler 
simulation to give the range rate and provides a delay 
equivalent to the slant range. Altitude calibrations 
can also be made. 


Antenna Alignment 

For certain systems, such as the AN/APQ-7, it is 
necessary to check the antenna for accurate align- 
ment with the radar system. Various methods of 
achieving this alignment have been effected using a 
signal generator test set and a portable synchroscope 
(which can be used conveniently at the aircraft). The 
procedure is greatly facilitated by use of a suitable 
test indicator, which is a selsyn unit with a calibrated 
dial to permit manual rotation of the selsyn rotor 
through a prescribed number of degrees. 

Shoran Test Equipment 

The maintenance of the Shoran system is some- 
what different from that of a microwave radar bomb- 
ing system, although the concepts of maintenance 
philosophy, design considerations, instruction litera- 
ture, and training still obtain. The Shoran test equip- 
ment consists primarily of a frequency meter accurate 
to within 0.2 per cent; a heterodyne frequency 
meter, accurate to within 0.05 per cent; a power 
meter, good to within 15 per cent; a signal generator 
(similar to the General Radio type 804), and an 
oscilloscope-synchroscope. 

Beacon Test Equipment 

Beacon test equipment is generally of three types, 
namely: that which provides a quick, overall check 
at the aircraft, for use with lightweight aircraft 
beacons; that which provides a more complete check, 
such as on the bench but which is still quasi-portable ; 
and that which is built into the beacon system as 
part of a permanent installation. 

A test set for performing quick, overall checks at 
the aircraft need only provide a pulsed r-f signal 
whose power output, which is monitored, is adjust- 
able so as to barely trigger the beacon; a means of 
measuring the beacon’s output power; and a cathode- 
ray tube on which the beacon code can be viewed. 
The actual requirements of such a test set are de- 
termined by the type of beacon under test. Thus, a 
test set for a quick check at the aircraft might have 
to provide paired r-f pulses of several fixed separa- 
tions (which can be selected by a switch, for example) 
of two levels: one directly from the output of the os- 
cillator (for purposes of calibration such as with a 
power meter) ; the other, at a different level ( — 20 db 
for example), for providing a signal which will enable 
the rejection of receivers whose sensitivity is below a 
prescribed amount. Also, a measurement of the fre- 
quency of the input and the output to within 0.5 me 


REST HI GTE I ) 



RADAR MAINTENANCE 


65 


might be required, as well as a means of measuring 
the beacon power output and beacon code. 

Test equipment for making more complete tests on 
beacons includes the following: (1) a signal generator 
test set — capable of measuring beacon power ouput, 
beacon receiver sensitivity, overall bandwidth, and 
frequency — which provides fixed output pulses of 
1 or 2 jusec duration and a pulse variable in duration 
from 0.2 to 5.5 fxsec that is delayable from 10 to 200 
jusec; (2) an oscilloscope on which the r-f pulse dura- 
tions can be measured accurately, using an r-f en- 
velope viewer; (3) an r-f dummy load; (4) a voltage 
divider; (5) a pressurization pump; (6) a means of 
making standing wave ratio measurements and, 
possibly, (7) a spectrum analyzer. The signal gen- 
erator test set and the oscilloscope are part of the 
permanent installation. 

Thus, for checking a beacon, it is generally de- 
sirable to check the minimum r-f signal on which the 
beacon will trigger, the pulse durations to which the 
beacon responds, beacon receiver sensitivity, and the 
beacon code on an oscilloscope capable of displaying 
the pulse durations accurately. 


GPI Test Equipment 

The ground position indicator system [GPI] (Chap- 
ter 9) w'as designed so that no special items of test 
equipment would be required. The general items of 
test equipment include a test calibrator for the range 
unit, such as one with a circular precision sweep, a 
portable oscilloscope-synchroscope for use at the air- 
craft and one providing more accurate displays for 
use at the bench, a bellows for applying constant 
pressure to the airspeed- meter to check the inte- 
grators, a means of compass alignment, a stop watch, 
and the other items of test equipment, such as the 
signal generator test set. 

Conclusion 

The above brief discussion of test equipment serves 
only to illustrate the additional maintenance prob- 
lems that result in changing from a search to a bomb- 
ing radar. The fundamental radar maintenance 
problems described in Section 5.1 still exist for bomb- 
ing systems and must be considered in the design of 
any radar set. 


ISTRICTED 


Chapter 8 


BOMBING COMPUTERS FOR RADAR MAPPING SYSTEMS 


8.1 THE COMPUTER PROBLEM 

8.1.1 Introduction 

In Chapter 7, a radar mapping system was defined 
as a radar equipment which presents on a scope a 
map of the terrain below and around the aircraft in 
which it is installed. Similarly, a bombing computer 
for a radar mapping system is defined as a computer 
which determines the time, place, and heading at 
which to release bombs in order that a target desig- 
nated on the radar scope will be hit. The designating 
index usually consists of the intersection of a range 
mark and an azimuth mark which is made to coin- 
cide with the target signal on the radar scope. Several 
such bombing computers are analyzed in the present 
chapter. 

Three general classes of computers are considered : 
nonsynchronous computers, semi-synchronous com- 
puters, and synchronous computers. In this classifi- 
cation, synchronous computers are defined as those 
computers which, by nature of their design, obtain 
data pertinent to release by a technique of continuous 
synchronization of a computer index with the target. 
These are contrasted with nonsynchronous com- 
puters which determine the release point, designate 
this on the plan position indicator [PPI] by a com- 
puter index, and require steering of the aircraft until 
the index and target coincide. The semi-synchronous 
computers make a partial synchronous and partial 
impact prediction solution by attempting to establish 
a computer index rate which keeps the index in syn- 
chronism with the target. The computer index, as 
discussed here, would be the cross hairs of the sight- 
ing telescope for a visual bombsight, or for a radar, 
the intersection of an azimuth mark and a range mark 
on a radar scope. 

The synchronous computer types are particularly 
satisfactory for radar since they permit maximum 
use of information about the target before the target 
is at such close range to the aircraft as to become 
badly distorted and confused. Most of the computers 
discussed herein fall into the synchronous or semi- 
synchronous classification. However, the first radar 
computer designed for over-land bombing use was of 
the nonsynchronous, impact-prediction type. The 
production versions of this computer were incorpo- 
rated in the AN/APS-15 and AN/APQ-13 radar 
equipments. Another type of impact-prediction com- 


puter was used with the AN/APQ-7 (Eagle) equip- 
ment. These computers are discussed in Section 8.2.1. 

The next step taken to improve radar bombing was 
to supply radar range and angle information to the 
Norden visual bombsight in an attempt to reap the 
benefits of a Norden synchronous computer. Since 
this method also pointed the visual telescope at the 
target by means of the radar, any break in clouds or 
in the smoke covering the target could be used for 
visual bombing. Furthermore, since the Norden sight 
was synchronized with the target by the procedure, 
its bomb-release mechanism was used instead of de- 
pending upon the information from the radar picture 
which became badly distorted as the aircraft ap- 
proached the bomb release point. This semi-syn- 
chronous technique was developed as an interim 
measure pending the advent of fully synchronous 
schemes. Other interim computers were the Visar 
and Nosmo which are completely synchronous in 
range but not in azimuth. 

The list of completely synchronous computers in- 
cludes the AN/APQ-5, the AN/APA-5, the MX-344, 
the ground position indicator [GPI], the AN/APQ-10, 
and the universal bombsight [UBS] computers. 8181 In 
this chapter the AN/APQ-5, AN/APA-5, and 
MX-344 computers are described and GPI is dis- 
cussed in Chapter 9. This means that w r ith one ex- 
ception, only the synchronous computers that were 
developed at the Radiation Laboratory will be dis- 
cussed in this book. The single exception is the 
MX-344 computer which w r as designed at Bell Tele- 
phone Laboratories. (The AN/APQ-10 and UBS 
were also developed at Bell Telephone Laboratories.) 
Of the computers discussed in the following sections 
only GPI attempts to solve all four problems in 
bombing, namely, navigation to the target area, 
identification of the target, computation of the re- 
lease point, and steering to the release point. The 
others calculate the release point and steer to the 
release point but depend on radar pilotage or other 
means for navigation, and visual (optical or radar) 
recognition for identification. 

The impact-predicting and semi-synchronous sys- 
tems are all limited to straight-line, non jinking bomb 
runs with the additional possibility of range-only 
offset. (In offset bombing, a bomb run is made by 
using a radar or optical reference point w’hich is lo- 
cated at a known distance and direction from the 


66 



NONSYNCHRONOUS COMPUTERS 


67 


target to determine the release point for the target). 
Of those classed as synchronous computers, the 
AN/APQ-5, AN/APA-5, and MX-344 are limited to 
straight-line nonjinking bomb runs. The MX-344 is 
also capable of offset bombing unlimited in azimuth 
but limited in range to approximately 10 miles. How- 
ever, the accuracy falls off when azimuth angles 
greater than 60 degrees from the line between target 
and reference point are used. By the use of an attach- 
ment, it is also possible to use the AN/APA-5 for 
offset bombing limited in range to 5 miles and in 
azimuth to 45 degrees from the target to reference 
line. GPI provides an offset solution (unrestricted in 
azimuth) out to ranges of approximately 10 miles 
from the target in any direction. 


8.2 NONSYNCHRONOUS COMPUTERS 

8.2.1 Impact-Predicting Computers 

The first radar blind bombing equipments con- 
sisted of modified search radar sets to which had been 
added a precision range unit and a simple bombing 
computer. The target was identified on the PP1 and 
the aircraft was steered toward the target by observ- 
ing it on the PPL The bombing computer and range 



Figure 1 . H2X drum computer. 

unit were used to create a bomb release circle on the 
PPL This circle indicated the slant range from the 
target at which the bomb was to be released in order 


to score a hit. Accordingly, the bombs were released 
manually when the target reached the bomb release 



Figure 2. Diagram of H2X drum computer. 


circle. These computers did not provide any means 
for determining drift or ground speed. Those im- 
portant factors must be determined by other 
means. 52d Although this type of radar bombing has 
exhibited relatively low accuracy, it played an im- 
portant role in the war against Germany and is de- 
scribed here principally because of its role in the 
development of radar bombing. The original bombing 
radar set was the British H2S equipment operating 
with 10-cm radiation. The principles of that equip- 
ment were embodied in the American 3-cm H2X 
radar equipments. The Eagle (AN/APQ-7) equip- 
ment adapted those principles to a high-resolution 
radar system. Several typical impact-predicting 
bombing computers will be discussed in this section. 


H2X Drum Computer 

The term H2X includes two particular radar sys- 
tems that were produced in large quantities. These 
are the AN/APS-15, w r hich was widely used by the 
8th and 15th Army Air Forces for the bombardment 
of German-held territory during 1944 and 1945, and 
the AN/APQ-13, which was extensively used by the 
20th Army Air Force against the Japanese in the 
Pacific Area during those same years. The original 
bombing computers for both systems were identical 
and the circuits of their range units were similar. The 
AN/APS-15 was used in this original form by the 
8th AAF in England during the first part of 1944 
but the AN/APQ-13 was not widely used until the 
h + b computer chart, described later, was incor- 
porated. 

The H2X drum computer is illustrated in Figures 


i 


RESTRICTED 


GROUND SPEED IN MILES PER HOUR ALTITUDE IN FEET ALTITUDE IN FEET 


68 


BOMBING COMPUTERS FOR RADAR MAPPING SYSTEMS 




SLANT RANGE NAUT MILES 



l< KiURE 3 A, B, C. Charts used on H2X drum computer. 




STRICTED 



AVERAGE BOMB- NO DELAY-TRUE GROUND SPEED ALTITUDE IN NAUT MILES ALTITUDE IN NAUT MILES 




NONSYNCHRONOUS COMPUTERS 


69 


1 and 2. The range knob rotates a cylinder or drum 
behind a fixed range index (parallel to the axis of the 
cjdinder) . The range knob also turns a potentiometer 
which provides control voltage for a variable range 
delay circuit in the range unit. The altitude knob 
on the drum computer moves an altitude index in 
front of the cylinder in a direction parallel to the 
axis of the cylinder. A potentiometer is also turned 
by the altitude knob, and this provides control volt- 
age for a variable altitude delay circuit in the range 
unit. Both delay circuits begin timing when a trans- 
mitted pulse is started. At the end of the variable 
altitude delay, the PPI sweep is started. Twelve 
Msec later (to allow for the slow starting PPI sweep), 
an altitude marker is produced on the A scope. Be- 
cause of a minimum altitude delay, the altitude 
marker cannot be set to less than about 10,000 ft. 
At the end of the variable range delay, a bomb re- 
lease marker is displayed. The bomb release marker 
appears on the PPI as a concentric circle. 

A chart is wrapped around the drum to provide 
computing scales. Several different charts used with 
this computer are shown in Figure 3. Moving hori- 
zontally (parallel to the longer edges) on any one of 
these charts is the equivalent of turning the range 
knob, and moving vertically (parallel to the shorter 
edges) is equivalent to turning the altitude knob. 

Only two operations are required to set up the 
proper bomb release circle on the PPI. First, the 
altitude knob is rotated until the altitude marker just 
touches the first ground return on the A scope. When 
this is done, the altitude cursor will indicate on the 
vertical scale of the computer chart, Figure 3 A or B, 
the absolute altitude at which the airplane is flying. 
Alternatively, if the absolute altitude is known by 
other means such as a barometric or a radio alti- 
meter, the altitude cursor may be set to that value 
directly. This first operation automatically sets in 
the proper amount of sweep delay to close the alti- 
tude circle on the PPI. The second operation consists 
of turning the range knob until the intersection of 
the range and altitude cursors falls over the proper 
ground speed line. This positions the bomb release 
circle on the PPI. The correct ground speed should 
be determined from navigational data collected en- 
route. 

Because the resolution of the radar presentation is 
much poorer than that afforded by the optics of a 
visual bombsight, the overall bombing accuracy to 
be expected by the impact-predicting radar bomb- 
sight is accordingly poorer. For this reason, several 


simplifying assumptions were made in the design of 
the chart for the drum computer, thus allowing the 
operator more time to devote to the difficult problem 
of target identification. The variation of trail with 
bomb type was neglected and calculations for trail 
and time-of-fall based on the ballistics of the AN/ 
M-43 500-lb general purpose bomb were incorporated 
in the chart. This was the bomb most widely used for 
strategic bombing in Europe. The ballistics of the 
heavier bombs did not differ too much from these 
and correction factors were later supplied for use 
with the AN/M-38A2 100-lb practice bombs used for 
training. The assumption that airspeed was equal to 
ground speed introduced a small error in trail and 
completely ignored the effect of cross trail. Field ex- 
perience has shown the wisdom of these assumptions 
since errors introduced by them are small in com- 
parison with the actual combat bombing errors. 

The discussion of range measurement in Section 
7.2.1 indicates that several sources may contribute 
to errors in radar range measurements. These errors, 
if sufficiently large, will be reflected in the overall 
bombing accuracy of the equipment. This was be- 
lieved to be true in the use of H2X equipments, and 
a modification was made to the computing method 
just described. The modification serves to reduce the 
effect of some of the errors and is explained in the 
discussion of the h + b technique below. 

h + b Technique 

One of the problems encountered in the use of the 
H2X equipments was that of maintaining proper 
calibration of the altitude and range delay circuits. 
Not only was the process of calibrating these circuits 
time-consuming, but also their constants were ob- 
served to change when they were flown to high 
altitudes. The error (A R) in measuring a slant range 
R is 

AR = E 0 + RE S (1) 

where E 0 is the zero error and E s is the slope error. 
This is true for both the altitude delay circuit and the 
range delay circuit, both of which contributed to the 
bombing error. 

The h + b technique is one in which the bomb re- 
lease range is considered as a slant range, OR, equal 
to the sum of the altitude, h, and an additional dis- 
tance, b, as shown in Figure 4. This shows how the 
h + b technique reduces any bombing error caused 
by an inaccurate calibration of delay circuits. The 
distance b varies with bomb type, altitude, airspeed, 


70 


BOMBING COMPUTERS FOR RADAR MAPPING SYSTEMS 



ORIGINAL H 2 X COMPUTER 

h = OA = true altitude 
OR = true slant range to target 
b = OR - h 

RRx = bombing error with original H 2 X computer 
RR 2 = bombing error with h + b computer 

Note. In Figure 4. 
Figure 4A, B. Reduction of 


h +■ b COMPUTER 

E u = zero error of range circuit 

E h = slope error of range circuit (fraction of true range) 
hE s = slope error for range h 
bE s = slope error for range b 

hE 0 should be hE s . 
ing error by use of h + b technique. 


and wind. Both h and b are measured by the same 
delay circuit. Referring to Figure 4A, RRi is the 
bombing error which would occur with a computer 
of the original H2X type as a result of assumed values 
of zero and slope errors in the range delay circuits. 
The bombing error shown in that figure is based 
upon the assumption that the true altitude was set 
into the computer. If the altitude used was obtained 
from the radar, zero and slope errors in the altitude 
delay circuit would cause an additional bombing 
error which might be either additive or compensative. 
In Figure 4B, Rh 2 represents the smaller bombing 
error which would result from the same zero and slope 
errors if the h -f- b principle is applied to the H2X 
comp uter . Note that RR 2 is considerably shorter 
than RKi. A second order error would be introduced 
by the use of an incorrect value of h in determining 
the value of b but this has only a negligible effect. 

Modified H2X Drum Computers 
The AN/APS-15 drum computer was modified to 
incorporate the h + b principle described above by 
adding a slant-range = altitude (or 6 = 0) line to the 
computer chart. This line, as shown in Figure 3B, is 
just to the left ol, and almost parallel to, the system 


of ground speed lines. Using such a chart, the pro- 
cedure for setting up the bomb release circle is as 
follows. With the altitude cursor set near the bottom 
of the scale and with a fast sweep (short range) on 
the PPI to create a large diameter altitude circle, 
the range knob is turned until the bomb release circle 
just touches the altitude circle. This evaluates the 
altitude, h. This altitude is transferred to the altitude 
scale of the computer chart by turning the altitude 
knob until the intersection of the range and altitude 
cursors lies over a point on the slant range = altitude 
line. Turning the range knob to place the intersection 
of the range and altitude cursors over the proper 
ground speed line on the computer chart completes 
the operation required to set up the bomb release 
circle. 

With the modification described above, it is de- 
sirable to have the altitude delay circuit adjusted 
with a small negative error in the zero setting in order 
to create PPI presentation with a small diameter 
altitude circle when the procedure of setting up the 
bomb release circle is completed. It is usually pref- 
erable, however, to completely divorce the altitude 
knob from the potentiometer controlling the PPI 
sweep delay circuit. The potentiometer may then be 


\ 


STRIOTED 

MiKBinrU'i' " 1 


NONSYNCHRONOUS COMPUTERS 


71 


used independently to adjust the PPI sweep delay. 
A computer of this type was designed for use with 
the AN/APS-15. 

The original AN/APQ-13 drum computer chart 
was replaced by the one illustrated in Figure 3C in 
order to make use of the h -f b technique in a slightly 
different manner from that described above. This 
was done before the AN/APQ-13 was put into large- 
scale use. The use of the intersection of the range and 
altitude cursors as an index was made completely 
independent of the altitude delay circuit. The hori- 
zontal scale at the bottom of the computer chart is 
an altitude scale calibrated in thousands of feet. 
The vertical scale is a ground speed scale calibrated 
in miles per hour. Diagonal lines extend the altitude 
scale across the ground speed scale for those altitudes 
between 15,000 ft and 35,000 ft. To use this chart 
the altitude h is measured on the altitude scale on the 
bottom of the chart by matching the bomb release 
circle to the altitude circle on the PPI. Setting the 
range index over the intersection of the diagonal 
altitude line and the proper ground speed line adds 
the necessary range increment b to produce on the 
PPI a bomb release circle of a diameter corresponding 
to the slant range, h -f b, at which bombs should be 
released. 

Dial Computer 

The AN/APS-15A Dial Computer was designed to 
overcome, wherever possible, the complaints that 
arose out of the early use of the original H2X drum 
computer. The principal objectives in this design were 
the incorporation of the h + b technique to reduce 
the effect of range circuit errors, and the provision 
of convenient scales which would enable the operator 
to make more precise settings. 

A photograph of the dial computer is shown in 
Figure 5. The altitude knob, which drives the arm of 
the potentiometer controlling the range delay circuit, 
(note this is the range rather than the altitude delay 
circuit) can be turned only if the ground speed knob 
is set at the zero position. The altitude scale indicates 
slant range up to 90,000 ft, in 100 ft intervals. The 
ground speed knob rotates the body of the same 
potentiometer, thus adding an increment of range to 
that set in by the altitude knob. Twelve ground speed 
scales which cover the possible combinations of three 
groups of bomb types and four ranges of indicated 
airspeed, can be inserted. The type of airplane in 
which the equipment is installed determines the indi- 
cated airspeed range, and thus reduces to three the 


number of scales required for any particular installa- 
tion. Bomb types were divided into three groups: 
light, medium, and heavy; and ground speed scales 
were based on the ballistics of the AN/M-38-A2 



Figure 5. AN/APS-15A dial computer. 

(100-lb) practice bomb; the AN/M-43 (500-lb) gen- 
eral purpose bomb; and the AN/M-34 (2,000-lb) 
bomb. In general, the use of a different ground speed 
scale for each group of bomb types was not justified 
in terms of the accuracy of aiming possible with the 
AN/APS-15 PPI presentation, but it was believed 
that the availability of such scales might have a 
favorable psychological effect on the operators. No 
addition to the operator’s duties during flight would 
result because the proper scale could be placed in the 
computer when the bombs were loaded into the air- 
plane. A third knob (sweep timing) controls the 
starting time of the PPI sweep over a range corre- 
sponding to 6 miles before the transmitted pulse to 
6 miles after the transmitted pulse. 

The operating procedure to set up the proper bomb 
release circle with the AN/APS-15A dial computer 
consists of two steps. With the ground speed knob at 
zero the altitude is determined by matching the 
bomb release circle with the altitude circle on the 


RESTRICTED ' 



72 


BOMBING COMPUTERS FOR RADAR MAPPING SYSTEMS 


PPI. (The altitude scale now indicates the height 
of the airplane above the terrain.) The ground speed, 
which was determined from navigational data, is 
then set into the ground speed knob, thus adding 
the range increment b to the altitude h which cor- 
rectly positions the bomb release circle on the PPL 

Factors Contributing to Inadequacy of Impact- 
Predicting Computers 

The poor results usually obtained with the impact* 
predicting computers are caused by certain funda- 
mental deficiencies of the H2X radar equipments as 
well as by tactical considerations. An example of the 
latter was the division of responsibility between the 
optical bombardier and the radar operator which oc- 
casionally led to confusion during the bombing run. 
This confusion was intensified by the fact that final 
decision as to whether optical bombing or radar 
bombing would be most effective in many cases 
could be made only after the bombing run was well 
under way. 

No accurate direct method of measuring ground 
speed or drift is provided. Although good naviga- 
tional procedure includes frequent calculation of 
those quantities, the tension and excitement of bat- 
tle just before and during the bombing run often 
causes errors in such calculations. An automatic 
means of determining and setting in those factors 
would be a valuable asset. 

The second major technical deficiency which con- 
tributed to the inadequacy of impact-predicting 
bombing computers was the distortion of the PPI 
presentation at ranges near the bomb release range. 
This distortion made the determination of the time 
at which the target crossed the bomb release circle 
extremely difficult. Several factors described below 
contribute to that deficiency: 

L The ratio of ground range to slant range 
changes very rapidly with range at short ranges and 
therefore causes severe distortion near the center of 
the PPI. This range distortion, discussed in Section 
7.2.1, is particularly great at the high altitudes from 
which bombing is usually done. 

2. The H2X antenna was deficient in two ways. 
The uneven vertical radiation pattern caused the 
sensitivity of the radar system to fluctuate with 
range in such a manner as to create concentric rings 
of bright and dim response on the PPI. (See Figure 
4 A, Chapter 7.) This required the operator to make 
frequent adjustments of the receiver gain and an- 
tenna tilt angle toward the end of the bombing run. 


The second deficiency was the increase in the 
azimuth beamwidth of the antenna at large depres- 
sion angles which impaired the azimuth resolution 
near bomb release range. 

3. The radar returns from the target area change 
considerably with aspect, thus changing the ap- 
pearance on the PPI of the target area. This change 
of appearance is particularly great near the bomb 
release range. 

The importance of the above deficiencies of the 
H2X with an impact-predicting computer was 
realized soon after that equipment was introduced 
into combat use by the 8th and loth Army Air 
Forces. An attempt to overcome some of them re- 
sulted in the coordinated bombing technique de- 
scribed in Section 8.3.1. 

8.3 SEMI-SYNCHRONOUS COMPUTERS 

8 . 3.1 Semi-Synchronous Tie-in of 
Radar to Optical Bombsight 

As previously stated, the two major deficiencies of 
the radar bombing procedure using an impact-pre- 
dicting computer are the deterioration of the radar 
presentation near the bomb release range and the di- 
vision of responsibility between the radar operator 
and the optical bombardier who pursue independent 
bombing functions. The coordinated bombing pro- 
cedure was developed to alleviate these deficiencies 
without waiting for redesign of the available radar 
and optical bombing equipment. This method was 
first used operationally and with marked success by 
the 8th AAF in bombing operations just prior to the 
invasion of the Normandy coast in June 1944. 77a The 
15th AAF also adopted this procedure at about the 
same time. 

The Norden bombsight has an inherent accuracy 
many times better than that of the H2X equipment 
but is useful only when the target is unobscured by 
darkness, clouds, fog, or smoke. The H2X equip- 
ment, although much less precise, is not affected by 
cloud or smoke cover. As has been previously stated, 
it was desirable to bomb optically whenever possible 
and for that reason the decision between optical 
bombing and radar bombing was often deferred until 
the end of the bomb run, which resulted in much con- 
fusion. Moreover, the radar presentation is con- 
siderably better at ranges just a few times greater 
than the bomb release range than at the release range 
so that if bombing information could be obtained at 


ESTRICT 


SEMI-SYNCHRONOUS COMPUTERS 


73 


the greater range, the accuracy of blind bombing 
would be improved. 

The coordinated bombing procedure has been 
termed semi-synchronous because it involves point- 
to-point synchronization of the tracking mechanism 
of the Norden bombsight from radar data. The radar 
operator informs the Norden bombardier of the time 
when the radar presentation of the target on the PPI 
is at certain definite points which correspond to 
specific optical sighting angles. Although this is in- 
ferior to the method of direct connection between the 
H2X radar and the Norden bombsight afforded by 
the AN/APA-46 and AN/APA-47 (Nosmo and 
Visar) attachments (see Section 8.3.2), it required 
only a simple modification of the H2X drum com- 
puter chart and was therefore immediately available 
for combat use on a large scale. 

The only essential equipment change was the addi- 
tion to the drum computer chart of a series of lines 
converting seven selected sighting angles to slant 
range as a function of altitude. The sighting angles 
chosen were 70 (maximum for Norden bombsight), 
68, 65, 62, 58, 53, and 46 degrees. These represent ap- 
proximately equal intervals of ground range which, at 
25,000 ft altitude, are about 1 mile in length. Actu- 
ally, new computer charts were prepared which in- 
cluded these sighting angle lines and also a family of 
ground range lines, which were helpful for ground 
speed measurements, as well as a (slant range = alti- 
tude) line to make possible use of the h + b technique 
described in Section 8.2.1. That chart is illustrated 
in Figure 3B. 

The same coordinated bombing procedure was used 
for both visual and radar bombing runs and this 
avoided last minute confusion in borderline cases. 
Sighting angle data were transmitted from the radar 
operator to the bombardier, via the interphone. Al- 
though the procedure requires a high degree of co- 
ordination between those two persons, it was far 
superior to the earlier nonsynchronous bombing 
methods. 

Before reaching the start of the bombing run, the 
bombardier sets into the bombsight all preset data 
such as disk speed, trail, approximate drift, and ap- 
proximate ground speed, just as he would in a normal 
visual bombing run. He next sets the sighting angle 
index at 70 degrees and waits for the signal from the 
radar operator. The radar operator has set the range 
index of the H2X computer over the intersection of 
the 70-degree sighting-angle line and the proper 
altitude line. This creates a circle on the PPI which 


corresponds to the 70-degree sighting angle. At the 
coincidence of this circle and the target return, the 
radar operator signals the bombardier who immedi- 
ately starts the Norden bombsight motor. If the 
preset data and the first check point were absolutely 
correct, the procedure could end here. Actually, this 
is not true, and the additional check points are used 
to correct the rate and displacement knobs of the 
Norden bombsight. This effectively causes the tele- 
scope of the bombsight to track the target and per- 
mits quick visual correction if an opening in the cloud 
or smoke cover appears. Whether a visual correction 
is possible or not, the tracking mechanism of the 
Norden bombsight automatically releases the bombs. 

By the summer of 1944, when the AN/APQ-7 
bombing radar equipment was nearing production, 
the coordinated bombing technique had proved its 
superiority over the impact-predicting method. Ac- 
cordingly, a design change was made in that equip- 
ment in time to be used in combat by the 315th Wing 
of the 20th AAF. The modification took the form of 
a few circuit changes in the operator’s indicator and 
the addition of two small boxes. One of these, the 
control unit, had controls for altitude adjustment 
(pulse matching on the A scope) and a switch per- 
mitting the use of either the original impact computer 
or the external components for synchronization. The 
arrangement also had the provision that either the 
box with selector switch for sighting-angle check 
points (using the same angles as with H2X) or the 
Nosmo attachment (see Section 8.3.2) when avail- 
able, could be connected to the control unit. This 
arrangement was quite flexible in that three tech- 
niques were possible. In the rush of war, equipment 
design may be finished long before evaluation tests 
with prototype models are complete — thus this 
flexibility was highly desirable. 

In the case of AN/APQ-7, the advantages of the 
Norden coordination are less clear. Because of the 
higher resolution and improved antenna coverage of 
AN /APQ-7, the target image is more sharply defined 
and can usually be tracked to the bomb release 
marker, thus eliminating the primary technical ob- 
jection to the AN/APS-15 impact-predicting system 
that led to the development of radar-Norden coordi- 
nation. In the case of AN/APQ-7 it would appear 
that when complete radar conditions are foreseen, 
i.e., 10/10 clouds or night, there might be some ad- 
vantage in using the impact-predicting procedure and 
in avoiding the necessary high degree of coordination 
between radar operator and Norden bombardier. 


74 


BOMBING COMPUTERS FOR RADAR MAPPING SYSTEMS 


8.3.2 Continuous Radar Tie-in 
with the Optical Bombsight 

The check-point method of setting up the optical 
bombsight by using radar information, described in 
the previous section, provides only an approximate 
solution of the bombing problem. When the Norden 
bombsight is used as an optical bombsight under 
conditions of good visibility, the rate and displace- 
ment controls are continuously adjusted to keep the 
cross hairs on the target. In the check-point radar 
method, adjustments can only be made at a few 
(usually seven) fixed sighting angles since time does 
not permit a greater number of adjustments. It is 
clear that the bombing in the latter case will be less 
accurate than in the former, since errors in position- 
ing the bombsight are obvious only at the specified 
sighting angles. 80a 

This deficiency of the check-point method was 
soon recognized and a number of methods of supply- 
ing more continuous radar information to the optical 
bombsight were conceived. By the end of World 
War II, production designs and systems had been 
completed for two of these methods, namely Nosmo 
(AN/APA-46) and Visar (AN/APA-47). 78a 

Design Principles of Nosmo, Visar, and 
Nosmeagle 

Both Nosmo and Visar make use of the optical 
bombing computer mechanism (the rate end) of the 
Norden bombsight to solve for the correct bomb re- 
lease point. The rate end (see Section 6.2.1) which 
normally drives the optical mirror of the bombsight, 
is coupled instead to a suitable nonlinear potentiom- 
eter. A voltage from the potentiometer is fed to the 
range circuits of the radar set where it controls the 
position of the bright ring of the PPI that is used as 
a bomb release index for the original impact-predict- 
ing method. In this case, however, instead of estab- 
lishing one position on the PPI corresponding to the 
bomb release point, the operator adjusts the rate end 
to keep the bomb release mark continually touching 
the chosen target. In this way, radar sightings replace 
the optical sightings and the rate end operates in 
its usual manner to compute the bomb release point. 
It should be noted that the radar information is 
neither so continuous nor so accurate as optical 
sightings because of the lower resolution of the radar 
system and the slow rotation of the scanner. 

Since the application of this type of bombing 
computer to any radar mapping system is obvious, 


it is not surprising that plans were made to adapt 
them to all three of the common bombing radar 
systems, AN/APS-15, AN/APQ-13, and AN/APQ-7. 
However, as a result of the abrupt termination of 
World War II, only a very limited amount of opera- 
tional information on the performance of Nosmo or 
Visar is available. The testing carried out under the 
auspices of the AAF Board at Orlando, Florida, indi- 
cated that an improvement by a factor of about 2 
was obtained in the bombing results, especially over 
complex targets. The AAF Board tests were termi- 
nated at the end of World War II, since both Nosmo 
and Visar were interim methods of providing syn- 
chronous bombing using readily available compo- 
nents, and they would be eclipsed by the develop- 
ment of better bombing computers such as GPI. In 
spite of their short life, these tw'o computers served 
to emphasize the contributions that synchronous 
range tracking, pulse doppler ground track determi- 
nation, and a synchronized optical bombsight could 
make in improving radar bombing. 

Although both Nosmo and Visar employ the Nor- 
den rate end to solve the bombing problem, they 
differ markedly in other respects. Thus, when Nosmo 
is used, the radar operator adjusts an auxiliary 
Norden rate end to cause the bomb release circle to 
track the target on the PPI while the bombardier 
synchronizes the optical bombsight with the radar 
operator’s rate end by watching an indicating meter. 
In this case, either rate end may be energized to drop 
the bombs. On the other hand, when Visar is used, 
the optical bombardier tracks the target on a remote 
PPI, located in the nose of the airplane, by using the 
same controls that adjust the optical telescope of the 
bombsight. Thus, in the Visar method, the principal 
function of the radar operator is to keep the radar 
system in good adjustment and to assist the bom- 
bardier in identifying the target correctly on the PPI 
screen. The resultant advantage of Visar is that both 
the bombardier and the radar operator adjust con- 
trols with which they are familiar. The main disad- 
vantage of Visar is the necessity of training bom- 
bardiers in the often difficult task of interpreting the 
PPI radar picture. Visar also required that the 
bombardier be able to use either the radar PPI or the 
optical bombsight which is troublesome since light 
conditions are so different in the two cases. 

The second major difference between Nosmo and 
\ isar is the method of killing drift on the bombing 
run. When Visar is used, a reference azimuth line 
whose position is controlled by the drift knobs of the 


V [iKSTKKTKl/ 


SEMI-SYNCHRONOUS COMPUTERS 


75 


Norden sight is displayed on the PPI. By an adjust- 
ment of these knobs, the airplane heading is changed 
through the automatic flight-control equipment or 
the pilot’s direction indicator so that the ground 
track of the airplane passes through the target and 
the drift is killed. Nosmo, on the other hand, takes 
advantage whenever practicable of the innate ability 
of a radar system to determine the ground track of 
the airplane by an application of the pulse doppler 
phenomenon. 5 - 8 * From a knowledge of the present 
ground track which may be rapidly determined by 
this means, corrections in heading are given to the 
pilot to bring the ground track through the target. 
Even when Visar is employed, the ground track, and 
hence the wind, may be determined by the Doppler 
method prior to the bombing run and used as an aid 
in presetting the Norden sight. It is noteworthy that, 
in both Nosmo and Visar, cross trail (Section 6.1.3) is 
neglected, which is usually permissible for common 
types of bombs. 

With these exceptions, the technical design of both 
AN /APA-46 and AN / APA-47 is so similar that only 
the former will be described. 

Design of Nosmo (AN/APA-46) 

The design of the Nosmo radar bombing computer 
has been described in detail. 30 Only a brief description 
of some of the salient features will be undertaken 
here. As seen in the previous section, the Nosmo 
computer employs the Norden rate end to establish 
the correct release point and determines the correct 
bombing course by an application of the pulse 
doppler phenomenon. 

The three most important components of 
AN/APA-46 are illustrated in Figure 6, namely, the 



c- 264 /APA-46 


Figure G. Major Nosmo components. 

computer, the control box, and the servo-amplifier. 
In addition, an antenna attachment, a junction box, 


a comparator meter for the optical bombardier, and 
an attachment to the optical bombsight, are required. 

Pulse Doppler Course Determination. The first part 
of the bombing problem is determining the course 
that the airplane should fly. It is solved in the case 
of Nosmo by using the pulse doppler phenomenon. 
Nosmeagle, on the other hand, did not use this 
method of course determination. For a detailed de- 
scription of the doppler phenomenon, see Part V and 
the R. L. Technical Series. 52 Briefly, whenever the 
radar scanner is pointed along the ground track of 
the airplane, the appearance of the radar echo re- 
turned from the ground area is characteristically 
modified. Therefore, by moving the antenna slowly 
through the region in which the suspected ground 
track lies, and examining the radar echo, it is possible 
to determine the ground track of the airplane within 
±0.5 degree over land. In rough air such as that over 
mountains the accuracy will be less, and it is not 
yet possible to make an accurate determination of 
ground track over water. 52 

In order to position the antenna slowly and ac- 
curately, a servomechanism is used so that the set- 
ting of a knob on the control box (see Figure 6) de- 
termines the direction in which the scanner is pointed. 
In addition to the positioning feature, the servo- 
mechanism also provides for the display of a radial 
mark on the PPI to indicate the ground track of the 
airplane on the radar map after the radar mapping 
feature is resumed. The radial mark may be made 
either bright or dark at the discretion of the radar 
operator and the use of the dark line will be prefer- 
able over built-up areas. Since the ground track will 
change whenever the airplane is turned and also 
whenever the wind shifts, a new determination of 
wind will be required periodically and as often as a 
turn is made. If the PPI presentation is stabilized, 
turns of 10 or 15 degrees can be made without a re- 
determination of ground track, since the drift angle 
does not change appreciably. The need for repeated 
evaluation of ground track is a disadvantage of the 
Nosmo technique because radar mapping must be 
suspended for 20 or 30 sec while the ground track is 
being resolved. This disadvantage could be eliminated 
by automatically displaying the instantaneous ground 
track on the PPI even though the antenna is con- 
tinually scanning. The advantages to navigation 
and bombing of a radar display of the airplane’s 
instantaneous ground track require no emphasis. 

The doppler phenomenon might also provide an 
indication of the instantaneous ground speed. 52 Since 


•i 


ESTRIGTBD 


76 


BOMBING COMPUTERS FOR RADAR MAPPING SYSTEMS 


ground speed is a major factor in establishing a bomb 
release point, future radar bombing computers will 
probably make greater use of the doppler phenome- 
non in measuring ground speed as well as ground 
track. 

Rate Computation. The heart of the Nosmo scheme 
is the computer CP-17/APA-46, which consists of 
the Norden rate end (see Section 6.2.1), whose output 
shaft is coupled to movable arms of two potentiom- 
eters. One of these is a linear potentiometer of the 
same type that is attached to the rate end output 
shaft of the optical bombsight. The positions of the 
arms of both these linear potentiometers are con- 
trolled by the rotation of the output shafts of their 
respective rate ends. By impressing the same voltage 
across both potentiometers and making the differ- 
ence in the voltage at the potentiometer arms zero 
when read on the bombardier’s comparator meter, 
the optical bombsight can be synchronized with the 
Nosmo computer. 

The second potentiometer, which is coupled to the 
Norden rate end of the Nosmo computer, is a non- 
linear potentiometer. The voltage from the arm of 
the nonlinear potentiometer is connected to the radar 
range unit (see Section 7.2.1) and positions the bomb 
release circle on the PPI. When the correct calibra- 
tion has been made, the bomb release circle indicates 
the slant range corresponding to the altitude of the 
airplane and the sighting angle indicated on the rate 
end. The method by which this is accomplished 
follows. The rotation of the shaft of the rate end, 
and consequently the motion of the arm of the non- 
linear potentiometer, is proportional to the tangent 
of the sighting angle, that is, ground range divided 
by altitude. The nonlinear potentiometer is in series 
with a fixed resistor connected to ground potential. 
This resistor is so designed that the resistance from 
the arm of the potentiometer to ground potential is 
proportional to the secant of the sighting angle, i.e., 
slant range divided by altitude. If the sighting angle 
is zero (corresponding to a target directly below the 
aircraft) the arm of the potentiometer is at the end 
of the potentiometer connected to the fixed resistor. 
In this case, the voltage transmitted to the range unit 
corresponds to that of slant range equal to altitude. 
Therefore, by adjusting the voltage applied to the 
nonlinear potentiometer and fixed resistor network 
until the voltage across the fixed resistor brings the 
bomb release circle into coincidence with the altitude 
signal (see h + b technique, Section 8.2.1), the Nosmo 
computer is calibrated for that particular altitude. 


Once this procedure has been carried out, the bomb 
release circle on the PPI will indicate the slant range 
which corresponds to the particular sighting angle 
indicated by the Norden rate end for the particular 
altitude at which the computer was calibrated. 

When the computer has been adjusted in this 
manner, and the radar operator adjusts the rate end 
controls so that the bomb release circle stays in con- 
junction with the radar presentation of the target on 
the PPI, the correct sighting angle information is fed 
into the Norden rate end to permit it to calculate the 
bomb release point. In other words, Nosmo is one 
method of translating the slant-range information 
provided by the radar system into the sighting angle 
information for which the Norden bombsight was 
designed. 

Although this method of determining the release 
point has many advantages, it retains two of the in- 
herent disadvantages of the Norden optical sight as 
well as those inherent in the reduced resolution of the 
radar system. These are the necessity of a long 
straight bombing approach and the limitation of the 
sighting angle to angles less than 70 degrees, in order 
to synchronize the rate end. Improvements in anti- 
aircraft radar fire control and the use of proximity 
fuses will make obsolete any bombing computers re- 
quiring a straight and level approach. Moreover, the 
increased speed of future bombing aircraft will re- 
duce drastically the time on the bombing run, par- 
ticularly if the maximum sighting angle is only 70 
degrees. A 70-degree sighting angle corresponds to 
approximately 11 miles ground range for an airplane 
at 24,000 ft, while bombs are dropped at about 3 miles 
ground range at ground speeds of about 180 mph. 
Thus, the release point problem must be solved 
while the airplane is traveling a distance of 8 miles. 
When the ground speed of the airplane is 180 mph, 
8 miles corresponds to a time interval of 160 sec 
(which is acceptable). If the ground speed were 
tripled, the time available for synchronizing on the 
target w r ould be far from adequate. The obvious im- 
mediate answer to this difficulty is to modify the 
rate end so as to permit greater sighting angles. A 
mechanical modification to permit such an increase 
has been constructed at the Armament Laboratory 
at Wright Field. 

Nosmo and similar hybrid products of radar and 
optical bombing systems were developed as interim 
bombing methods. A better bombing computer would 
take advantage of the greater range and range ac- 
curacy that radar systems can provide as compared 




LOW-ALTITUDE BOMBING COMPUTERS 


77 


to optical systems. The AN/APA-5, MX-344, and 
GPI computers described in subsequent sections are 
all attempts to take advantage of these qualities of 
radar systems. While the Nosmo method of solving 
the rate problem has been superseded by these newer 
computers, the Nosmo method of determining a 
course to bring the ground track of the airplane 
through the target is novel and still useful. 

8.4 LOW-ALTITUDE BOMBING 
COMPUTERS 

8.4.1 General Principles of Low- 
Altitude Bombing Computers 

The tactical use of radar search systems employed 
in antisubmarine activities during the early part of 
World War II was limited by the lack of suitable 
radar bombing methods. It was evident that a low- 
altitude radar bombsight would find wide applica- 
tions not only against submarines but also in night 
attacks against enemy shipping. In the spring of 
1942, therefore, an experimental bombing attach- 
ment for basic search radars and the Norden sight 
was developed which, because of its low-altitude 
operational limit, was commonly called low-altitude 
bombing [LAB]. The basic features of this experi- 
mental model were later incorporated into production 
equipments, the AN/APQ-5 and the AN/APA-5, 
sometimes referred to as the LAB Mk I and LAB Mk 
II respectively. 

Actually the term LAB, as commonly used, de- 
notes systems derived from the Radiation Laboratory 
experimental LAB, but which are not necessarily 
limited to low-altitude operation. They are bombing 
attachments and require that the basic radar system 
furnish only the synchronizing trigger, video signal, 
and azimuth reference data. The normal operation of 
this basic radar and the Norden sight is not impaired. 
A list of the search radars which may be used in 
combination with LAB computers includes the 
SCR-517, SCR-717, AN/APQ-13, AN/APS-15, and 
AN/APS-10. Since LAB equipments obtain video 
signals from the basic radar, the resolving capabil- 
ities of these radars are a factor affecting the bombing 
accuracy and type of target against which the equip- 
ment may be employed. The low-altitude limit of the 
experimental LAB and the AN/APQ-5 generally 
confines their use to isolated targets. The AN/APA-5 
and the AN / APQ-5B contain provisions for extend- 


ing the operating altitude, permitting their use 
against any target which can be resolved on the 
bombardier’s indicator. 

Fundamentally, the operational principles asso- 
ciated with LAB are analogous to those of the 
Norden sight. This similarity is desirable for two 
reasons; first, the basic philosophy of the Norden sight 
has been well proved, and Second, the training of 
LAB bombardiers, who have had previous Norden 
sight experience, is simplified. 

As in optical bombing, the duties and responsi- 
bilities of the navigator and bombardier are clearly 
defined. The navigator guides the aircraft to the 
target area and the bombardier takes control of the 
plane only after sighting the desired target. This di- 
vision of responsibility allows the bombardier to con- 
centrate exclusively on the necessary preliminary 
procedures before the start of the bomb run, as well 
as to devote full time to the process of synchroniza- 
tion during the run. 

All LAB systems employ the same technique for 
steering to the release point, and furnish the bom- 
bardier with a similar type of indicator presentation 
during low-altitude operation. A B scope (see Chap- 
ter 7), mapping 10 miles in range and expanded in 
azimuth, is used for initially locating the target. 

During the tracking procedure, the target is viewed 
on a 1-mile expanded portion of the search sweep. 
Centered on this expanded indication is an azimuth 
and range cross hair. 

A potentiometer or selsyn tie-in between the radar 
scanner and the Norden azimuth stabilizer makes the 
azimuth cross hair correspond with the azimuth of 
the Norden telescope. The bombardier, by operating 
the azimuth stabilizer knobs in the usual manner, 
steers the plane to the proper release point by syn- 
chronizing the target on this azimuth cross hair. 
Figure 7 illustrates this method of steering and the 
target position for the conditions of this approach. 
The method of establishing a collision course is 
identical with that used in the Norden sight. Target 
drift with respect to the azimuth cross hair is more 
readily detected on the expanded sweep than on a 
conventional PPL The tie-in between the azimuth 
stabilizer and the radar scanner stabilizes the indi- 
cator presentation against yaw of the aircraft, but 
not against pitch or roll. 

The expanded portion of the search sweep used for 
target tracking purposes is continuously varied in 
range as the target is approached. The range rate and 
the displacement of this moving sweep are adjusted 


78 


BOMBING COMPUTERS FOR RADAR MAPPING SYSTEMS 


by knobs, similar to those on the Norden bombsight, 
until the target remains positioned on the range 
cross hair. If the range synchronization is exact, the 
target remains stationary on the bombardier’s indi- 
cator. With such an expanded stationary presenta- 
tion, the blurring of moving signals by the persistence 
of the indicator is eliminated. The expansion of the 
target area on the bombardier’s indicator aids in 



RANGE CROSS HAIR 
A' B’ C' 


A'. Scope presentation with aircraft at A. Aircraft heading 
and line of sight to the target coincide. Target appears centered 
on azimuth cross hair. B ' . Scope presentation with aircraft at B. 
Aircraft heading and line of sight same as at position A. Target 
is left of the line of sight and therefore appears to the left of the 
azimuth cross hair. C . Scope presentation with aircraft at C. 
Aircraft has been turned by azimuth course knob' until target is cen- 
tered on azimuth cross hair. Line of sight and aircraft heading 
still coincide. Aircraft heading is then changed by adjustment 
of drift knob through an additional approximated drift angle. 
Target remains centered on azimuth cross hair during adjustments 
of the drift knob because of stabilized line of sight. 


Figure 7. LAB method of steering and indicator 
presentation. 


positioning the tracking line and in identifying com- 
plex targets. The slant range velocity is determined 
by this range synchronization procedure and is en- 
tered into a range computer. The range computer 
positions a release signal at the proper slant range for 
the conditions in the bomb run. Bomb release is 
automatic, occurring when the slant range of the 
tracking line and the release signal are equal. 

The range computers are different in the various 
LAB systems and are discussed below. 


8.4.2 AN/APQ-5 

The basic principles of the experimental LAB were 
incorporated in the production of AN/APQ-5. 86 This 
equipment played a major role in night attacks on 
Japanese shipping in the southwest Pacific after its 
first combat introduction in the fall of 1943, and es- 
tablished an impressive combat record. 76a One re- 
port 71 revealed that during the 4-month period from 
December 1943 to April 1944, one squadron equipped 
with AN/APQ-5 sank 47 per cent of the total tonnage 
sunk by its Bomber Command and in doing so, 
utilized only one-fifth of the planes, bombs, and per- 
sonnel employed by that command in attacks against 
enemy shipping. The use of the AN/APQ-5 and 
a second version of this equipment, called the 
AN/APQ-5B, was continued by the Army Air Forces 
until the termination of hostilities. 

AN/APQ-5 Range Computer 

This section will be devoted to a description of the 
AN/APQ-5 range computer and to the solution of 
the tracking and release triangles. This computer was 
designed for operation at altitudes between 65 and 
2,000 ft. The range tracking procedure is not truly 
synchronous since the range synchronization pro- 
cedure measures rate of change of slant range (slant- 
range velocity), not true ground speed (rate of change 
of ground range). As a result, the slant-range velocity 
must be determined early in the run where the slant 
range and ground range are nearly equal. Once the 
range rate has been determined, any drift of the 
target from the range cross hair is corrected by use 
of the range displacement knob only. During the 
later part of the run, the bombardier must continu- 
ously reposition the target on the tracking line. If the 
target is lost because of fading, distortion, or sea re- 
turn, range errors will result because of the inability 
of the operator to position the target correctly on the 
range cross hair. 

The slant-range release distance is actually the 
ground range to the target at release ( V g T f ) plus a 
second distance which is a function of closing speed 
and altitude. Figure 8 shows the release triangle and 
includes a derivation of the slant-range release dis- 
tance in terms of altitude and closing speed. The 
voltages proportional to 



and ( k'Vgh 2 ) are obtained by means of a resistance 




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COMPLETELY SYNCHRONOUS COMPUTERS 


79 


computer whose basic circuit is shown in Figure 9. 
The resistance values of the basic circuits in this 
figure are proportional to the quantities h and V a as 
indicated. No derivation of the resultant voltages will 
be given, as the application of simple circuit theory 


RELEASE POINT 



S = slant range release distance 
Vg = ground speed 
h = altitude 

T f = time of fall = J /— , where q = 

S = Vh 2 + ( VgT f y = |//i 2 + 


acceleration of gravity 


VnTf = 


-I 

h 3 ' 2 


or S - V 0 T f ^ K- 


f — - h m +Vll 

K = constant) 


(approximation for 2,000 > h > 65 


h 3 ' 2 


S = (S - V.T f ) + V.T, = K ] ^ Wo + K'V'h" 

(K r = constant) 

Figure 8. AN/APQ-5 release triangle solution. 


will establish the validity of the expressions. The use 
of ganged potentiometers enables all altitude (h) 
and closing speed ( V g ) data to be entered into the 
computer with only one dial setting for each quantity. 
Nonlinear potentiometers are used to provide the ex- 
ponential functions of h. The output voltages from 
this computer, plus a third voltage, are combined and 
are used to position the electronic release index. This 
third voltage, a slant-range increment voltage, modi- 
fies the slant-range release distance to compensate 
for bomb trail or to permit dropping a number of 
bombs in train. 

The altitude is set into this computer on a dial 
before the bombing run. The closing speed, actually 
a measure of the slant-range velocity, is entered into 
the computer automatically by the process of rate 
synchronization. The slant-range increment is en- 


tered on a second dial and is obtained by reference 
to charts after V g has been determined. 



E( 


h 3 /2 

h^2+V g 


) 



E' (V g h V 2) 


Figure 9. AN /APQ-5 range computer circuits. 


8.5 COMPLETELY SYNCHRONOUS 
COMPUTERS 

8.5.1 AN/APA-5 

General Description 

The AN/APA-5, or LAB Mk II, is a completely 
synchronous general purpose bombing attachment 
capable of operating at altitudes from less than 
1,000 ft up to 30,000 ft or more." Although the com- 
plete altitude range is covered by two overlapping 
scales, the operational procedures on each scale are 
identical. All necessary data, obtained from standard 
aircraft instruments and bombing tables, are entered 
on computer dials before the run is begun. Once the 
bombardier has started the run, his only operation is 
to keep the target on his range and azimuth cross 
hairs. No further reference to tables is required. The 
process of range synchronization establishes the true 
ground speed which is automatically entered into 
the computer. The solution of the slant-range release 
distance is automatic and involves no approxima- 
tions. Range tracking is completely synchronous and 
extrapolated releases are possible from any point 
during the run. 

AN/APA-5 Operation 

The method of steering to the release point has al- 
ready been discussed in Section 8.4.1. 37 The range 


giu 


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80 


BOMBING COMPUTERS FOR RADAR MAPPING SYSTEMS 



to = time at which tracking is started 
V c = closing speed (velocity of airplane relative to target) 

Tr = trail (distance by which bomb lags airplane at time 
of impact) 

Tj = time of fall of bomb 

x, h, Vc, etc. are quantities in the computer which corre- 
spond respectively to x , h, V c , etc. 

puter dials are closing speed, 400 miles per hour; 
time of fall, 15 sec; trail (or lead), 500 ft; and alti- 
tude, 5,000 ft. These maximum values during high- 
scale operation are 400 miles per hour, 50 sec, 5,000 
ft, and 50,000 ft respectively. The exact solutions of 
the tracking and release triangles are obtained by two 
electrical triangle solvers which determine the vector 
sum of two quadrature sinusoidal voltages obtained 
from the two-phase generator. One of the sinusoidal 
voltages is proportional to the ground-range leg of 
the tracking and release triangles, the other to the 
altitude leg. These alternating voltages are combined 
in a resistance network and rectified, the d-c output 
of the two rectifiers determining the position of the 
tracking index and the release index on the scope. 

Before the run, the altitude, determined from 
standard aircraft instruments or the radar system, 
and the time of fall and trail, obtained from standard 
bombing tables, are entered on the computer dials. 
The range synchronization procedure automatically 
adjusts the closing-speed dial to the true closing 
speed, thereby eliminating the necessity for pre- 
setting this information. The target is synchronized 
on the range cross hair during the tracking operation 
by a single knob control which adjusts the closing- 
speed dial. By referring to Figure 10 and the discus- 
sion below, the theory of this single knob range track- 

i, 


x = ground range to target 
h = altitude of plane relative to target 
R = slant range to target 
Rr = slant release range 

Xo = ground range to target at time tracking is started 
t = time 



Figure 10. Operation of AN/APA— 5 range computer. 


scales depicted on the bombardier’s scope when used 
for search and tracking are 10 miles and 1 mile re- 
spectively during low-scale operation. These scales 
are identical with those of the AN/APQ-5. When the 
slant-range release distance exceeds 5,000 ft, the high- 
altitude scale of the range computer must be used, 
which changes the indicator sweep to 30 miles for 
searching and 3 miles for tracking. The operating 
scales are readily changed by switches on the control 
box. On both the low- and high-altitude scales, the 
operational procedure is the same. 

Figure 10 is a simplified diagram of the AN/APA-5 
computer and a diagram of the tracking and release 
triangles. Figure 11 is a photograph of an AN/APA-5 
installation in a B-24 airplane. During low-scale 
operation, the maximum readings of the four com- 


COMPLETELY SYNCHRONOUS COMPUTERS 


81 



CONTROL BOX 


INDICATOR 


STABILIZER SELSYN 


RANGE TRACKING BOX 


Figure 11. AN/APA-5 installation in B-24D. 


ing is apparent. At the instant range tracking is 
started by engaging the clutch between the constant 
speed motor and the tracking potentiometer, the 
initial ground range voltage (x 0 ) begins to decrease 
at a linear rate depending on the setting of the V c 
dial. At any time (t), the value of V c (t — to), the 


ground range distance traveled since the start of the 
tracking operation, must be equal to V c (t — to) if the 
target is to remain on the cross hair. If V c is not 
equal to V c , the target will appear to drift away from 
the cross hair. Repositioning the target with the 
range tracking knob equates V c to V c . Once V c is 


82 


BOMBING COMPUTERS FOR RADAR MAPPING SYSTEMS 


correctly determined, the tracking is completely syn- 
chronous during the remainder of the run. 

Evaluation 

The AN/APA-5 first became available in Febru- 
ary 1945, but because of the magnitude of the instal- 
lation and training program, was not introduced into 
combat before the end of the war. Several flight-test 
programs were conducted in the summer of 1944 
using a preproduction AN / APA-5 and an AN /APQ- 
13.42-44 a series of 40 bomb runs were made against 
a point target at each of three altitudes, 1,000, 5,000, 
and 15,000 ft. A summary of the results of these 
tests is shown in Table 1. 


Table 1 . AN/APA-5 test results against point target.* 


Altitude (ft) 

Range error 

Azimuth error 

Radial error 

1,000 

46 (ft) 

28 (ft) 

63 (ft) 

5,000 

33 (mils) 

19 (mils) 

40 (mils) 

15,000 

10 (mils) 

10 (mils) 

14 (mils) 


* All errors are probable errors about the target center, that is, 50 per 
cent of the total runs were within the error shown. 


Army Air Forces Board tests, using Army per- 
sonnel, were conducted against two complex targets, 
the H. J. Kaiser Company plant, Fontana, Cali- 
fornia, and the North American Aircraft Corpora- 
tion, Inglewood, California. Runs were scored by a 
photo-theodolite (see Section 13.1.2). A total of 55 
runs was made against the Kaiser plant at 15,000 ft 
altitude and 50 per cent of the total runs were within 
17 mils of the target center. Against the North 
American Aircraft Corporation from this same alti- 
tude, a probable error of 22 mils was obtained for a 
total of 50 runs. These errors were measured from an 
arbitrary target center, chosen before the runs were 
begun. 

The completely synchronous range computer, 
coupled with the 3-mile expanded sweep used during 
the high-altitude tracking operation, contributes 
greatly to the successful use of the AN/APA-5 
against complex targets. The radar pictures of com- 
plex targets have a tendency to break up and be- 
come badly distorted at the end of the bomb run. 
The synchronous range tracking of this equipment 
permits extrapolated releases from distances where 
this distortion is small. The wide operational limits 
of the AN/APA-5, plus the completely synchronous 
solution of the bombing problem, make it a versatile 
system adaptable for many tactical purposes. 


8.5.2 MX-344 Computer 

General Description 

Although the bombing computer to be described 
briefly in this section was developed by the Bell Tele- 
phone Laboratories, rather than the Radiation Lab- 
oratory, any discussion of computers for radar map- 
ping equipments would be incomplete without the 
inclusion of a section on the AN/APQ-13, Mod. II, or, 
as it was officially known, the MX-344 computer. 81a 

The MX-344 computer was designed to be op- 
erated by the regular radar operator, although it 
could conceivably be operated by the bombardier or 
by the navigator. Its size, however, practically rules 
out operation by the bombardier. The main controls 
of the computer are exactly analogous to those on 
the Norden bombsight as there are two sets of knobs 
(which may be double gripped), one set controlling 
ground range and ground speed, the other set con- 
trolling turn and drift. Bomb ballistics, in the form 
of time of fall, trail, and altitude, are set in on 
conveniently calibrated dials. 

Despite the similarity of controls, however, the 
MX-344 has two advantages over the Norden sight 
as a bombing computer. In the first place, tracking 
of the target may always be started when it is still 
approximately 15 miles away, since the tracking cir- 
cuits are dependent on time-to-go to the target rather 
than on the sighting angle to the target, as is the 
case for the Norden sight. This means that low- 
altitude runs are not handicapped by requiring the 
sight to be synchronized in a much shorter time than 
high-altitude runs. The second advantage of the 
MX-344 over the Norden and one of its most im- 
portant features is that it easily permits offset of the 
aiming point from the target. The limitations on the 
offset are that the aiming point must be within 10 
miles of the target and a course should be chosen 
which lies with + 60 degrees of the bearing of the 
target with respect to the aiming point. Other courses 
may be chosen, but the accuracy will be somewhat 
reduced. 

Among the strictly radar features of the computer 
is the provision of continuous sweep expansion so 
that the point under the cross hairs is held at a fixed 
position on the PPL This is of considerable impor- 
tance in high-speed planes where the motion of the 
returns on a high-persistence PPI can cause blurring 
of the picture, but the continuous sweep expansion 
may lead to certain tracking errors because of the 
variation in operator judgment when different sweep 


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COMPLETELY SYNCHRONOUS COMPUTERS 


83 


speeds are used. Two other desirable features are 
( 1 ) a longer double-gripping time constant between 
the range and rate knobs which is better adapted 
to radar than the Norden time constant of approxi- 
mately 10 sec, and ( 2 ) a method of giving sighting 
angle information to the Norden operator, so that 
even when he cannot see the target he can still syn- 
chronize his sight and thus be prepared to take ad- 
vantage of any break in the undercast. 

The operation of the MX-344 introduces no tech- 
nique essentially different from those involved in 
the operation of the AN/APA-5 or AN/APQ-5 com- 
puters except in so far as the radar operator is also 
expected to operate the computer. While navigating 
to the target area, the operator will set in the time 
of fall and the trail ratio for the briefed altitude and 
airspeed. He will also set in the offset range and 
bearing of the target from the aiming point, which 
will have been previously computed and given to the 
operator at the briefing, prior to the mission. On 
reaching the target area, the operator has two main 
tasks : identifying the aiming point and synchronizing 
the cross hairs on this point. The identification of the 
aiming point has been discussed in Chapter 6 . It is 
sufficient to say here that, since an offset aiming point 
may be used, a point easy to identify should be 
chosen. 

In order to synchronize the cross hairs on the aim- 
ing point, the operator must first set an accurate 
altitude into the computer. This is done with the 
altitude knob, by making a range circle coincide 
with the first ground return on the PPI. In order to 
facilitate this measurement, the computer is arranged 
so that when the operator pushes in on the altitude 
knob, the sweep speed of the PPI is increased with 
the result that the first ground return is out near the 
edge of the PPI. After setting in the altitude, the 
operator synchronizes the azimuth marker on the 
aiming point by using the turn and drift knobs, and 
then synchronizes the range circle on the target with 
the range and ground speed knobs. Actually, these 
last two operations are not independent and should 
be performed more or less simultaneously. This in- 
terdependence between the range and azimuth syn- 
chronization is undesirable, but is inherent in the 
MX-344 computer because of the method in which 
it solves the bombing problem. 

Azimuth Solution. The azimuth portion of the 
bombing problem is illustrated in Figure 12 . The 
true bearing to the aiming point is designated A and 
the true bearing to the target displaced by crosstrail 



is designated \J/, this latter being the desired ground 
track. The difference A — \J/ between these two 
angles is called a. The true bearing of a line extending 
from the aiming point to the target is /3, and the 
difference — (3) between this bearing and the de- 
sired track is 6. With a drift angle, 5, the desired 
heading of the plane, y, is ty — 8). 

The angle a is determined from the relationship 

Ra sin a — T r sin 8 = R 0 sin d 

where T R is the trail, Ra is the ground range to the 
aiming point, and R 0 is the offset ground range (from 
aiming point to target) . 

The desired heading 7 for the airplane is given in 
terms of the bearing to the aiming point by 

7 = A — a — 8. 

Figure 13 is a block diagram of the computer. 
Voltages are represented by solid lines, while me- 
chanical connections are shown as broken lines. An 
examination of the part of Figure 13 labeled “Azi- 
muth” indicates how the course of the aircraft is 
determined. A voltage that is proportional to the flux 
gate compass reading is connected to a torque unit. 
The torque unit is geared to a mechanical differential, 


84 


BOMBING COMPUTERS FOR RADAR MAPPING SYSTEMS 


TO PDI TO PPi 



Figure 13. Block diagram of MX-344 computer. 


whose second input is the turn control. The output 
from this differential is fed to another differential, 
the second input to which is the drift control. Thus 
the output of this second differential represents the 
instantaneous error e in the heading of the airplane 
and can be used to provide 'pilot's direction indicator 
[PDI] information and to control the automatic 
pilot. The mechanical differential, to which the 


bearing offset knob is connected, receives as a second 
input the rotation of the turn control and thus the 
output of the differential is seen to be equal to 6 
when the PDI reads zero. 

A steerable electronic marker is obtained from 
a cam mechanism. The cam is driven by a me- 
chanical differential whose output corresponds to 
S ~ W' — 7 4-a — e). The two inputs to the me- 




COMPLETELY SYNCHRONOUS COMPUTERS 


85 


chanical differential are the position of a selsyn motor 
repeating the antenna position S, and a second 
mechanical differential output corresponding to 
\p — 7 -f- a — e. (The means of obtaining the shaft 
rotation a will be discussed later.) Thus, when the 
azimuth marker is made to pass through the aiming 
point and the PDI reads zero, the aircraft will fly the 
proper course y. 

Vertical Triangle Solution. The vertical triangle 
to be considered is shown in Figure 14 where h is the 




R T 

Figure 14. Vertical geometry for MX-344 computer. 


altitude, Ra the instantaneous ground range to the 
aiming point, and pa the instantaneous slant range to 
the aiming point. 

Referring again to Figure 13, in the section labeled 
“ Vertical,” the ground speed knob controls a double 
potentiometer which is connected to a cos a potenti- 
ometer which converts ground speed into its com- 
ponent in the direction of the aiming point. The volt- 
age proportional to this component controls a vari- 
able speed motor which drives a shaft at the proper 
rate for tracking the aiming point. This shaft drives 
a sweep expansion potentiometer and also is one in- 
put to a mechanical differential, the other input being 
from the ground range knob. The output of this 
differential, which, when synchronized, gives con- 
tinuously the varying ground range to the aiming 
point, drives a square-law potentiometer for the 
vertical triangle solution, and is also used in the 
Alpha computer, as described below. The altitude 
knob drives a square-law potentiometer for the verti- 
cal triangle solution and also a dual potentiometer in 
the ballistic computer. 

The outputs of the two square-law potentiometers 
mentioned above are combined in a servo-amplifier 
which controls a servo-motor to make pa 2 = h 2 + Ra 2 , 


where pa 2 is derived from a third square-law potenti- 
ometer driven by the servo-motor. The motor also 
drives a linear potentiometer from which is derived 
the proper voltage for controlling the range mark 
delay circuit. 

Ballistic Computation. The ballistic computation is 
based on the following relationship at the correct 
bomb release time. 


Ra cos a + Rq cos 6 


VgTf — T R = 



— Tr 


where Rt is the instantaneous ground range to the 
release point. 

One of the input controls of the section labeled 
“Ballistic” in Figure 12 is proportional to the trail 
ratio, i.e., trail divided by altitude, and drives a 
double potentiometer which is excited by another 
double potentiometer driven by the altitude shaft. 
Thus the output of the first potentiometer is the 
trail T r . This output is fed into the bomb release 
mechanism and is also fed to a potentiometer, the 
arm of which is driven by the drift knob in the 
azimuth computer, and the output, T R sin b, (cross 
trail), is fed to the Alpha computer. 

The time-of-fall input to the ballistic computer 
drives a potentiometer, the voltage on which is 
proportional to the ground speed. The output 
(dR T /dt)T f is fed to the bomb release mechanism 
which evaluates the sum of the various input volt- 
ages. The other two inputs to this mechanism, R Q cos 
a and R A cos 6, are obtained from the Alpha com- 
puter as described below. 

Alpha Computation. As described previously, a is 
determined from the relationship, 

Ra sin a — T R sin b = R 0 sin#. 

The solution is performed by a servomechanism, the 
inputs to which are the three terms of the equation 
and the output is a shaft rotation proportional to a. 
Ra sin a is obtained by applying a voltage pro- 
portional to Ra to a sinusoidal potentiometer driven 
from the output shaft of the servo. T R sin b is ob- 
tained from the ballistic computer as described 
above. R 0 sin 6 is obtained by applying a voltage 
proportional to Ro to a sinusoidal potentiometer 
driven by the 6 shaft from the azimuth computer. 
The range offset knob drives a dual potentiometer 
to supply the Rq voltage, and the Ra shaft from the 
vertical computer does the same for the Ra voltage. 

The other outputs from the Alpha computer, R 0 
cos 6 and Ra cos a, are derived from the 6 and a 


86 


BOMBING COMPUTERS FOR RADAR MAPPING SYSTEMS 


potentiometers by using another sliding contact on 
each, 90 degrees away from the sine contact. 

Evaluation 

The MX-344 represented a definite advance in 
bombing computer design. Although it was late in 
appearing on the scene, it would have seen extensive 
and valuable use in the Pacific theater had not the 
war ended just when it did. Considering that approxi- 
mately 70 per cent of the bombing done by the 20th 
Air Force was radar bombing and considering all the 
shortcomings of even the sighting angle extension of 
the impact-predicting computer, the value of the 
MX-344 was very evident. 

The fact that the MX-344 provides offset features 
should also be considered in evaluating the computer. 
Although at first glance the offset feature might ap- 
pear to be the most important feature of the com- 
puter, several aspects of the problem must be con- 
sidered. In the first place, although at the time of 
the Japanese surrender it appeared that most of the 
targets remaining to the 20th Air Force could only be 
attacked by radar by the use of an offset aiming 
point, it was not demonstrated that these targets 
had satisfactory offset aiming points. Indeed, our 
knowledge of the value of offset bombing is based 
almost entirely on theory. Secondly, the MX-344 in- 
creases the maintenance problem. Not only is the 
computer itself complex, but also in order to obtain 


satisfactory results with it, the flux gate compass 
must be maintained at a high degree of accuracy. 
Indeed, the compass is most probably the limiting 
factor on the bombing accuracy that may be ob- 
tained. The compass problem is discussed in Chap- 
ter 9. 

Some improvements in the MX-344 are possible. 
For example, the single knob range tracking, such as 
used in the AN/APA-5 or the GPI, appears to be 
definitely superior to the double-gripping arrange- 
ment of the MX-344. Likewise, some provision 
might have been made in the MX-344 to utilize the 
pulse doppler method of drift determination, but it 
should always be remembered that the MX-344 was 
essentially an interim computer and the speed with 
which it had to be designed and built ruled out many 
features which might have been desirable. 

By August 1945, the theater installation of sev- 
eral MX-344 computers had been completed, and 
the theater reaction was definitely favorable despite 
the bulk and the additional burden it put on the radar 
operator and maintenance man. Although complete 
information concerning the accuracy of the com- 
puter on point and complex targets is not available, 
preliminary tests without offset indicate an improve- 
ment of at least a factor of 2 over the sighting angle 
method of using the impact-predicting computer. The 
results obtained using offset will depend to a large 
extent on compass maintenance. 


RESTBIC 



Chapter 9 


THE GPI BOMBING AND NAVIGATION COMPUTER 


9.1 THE FUNDAMENTAL GEOMETRY 
OF THE GROUND POSITION 
INDICATOR [GPI] 

Ground 'position indicator [GPI] is a term com- 
monly used to denote systems which indicate con- 
tinuously the present position on the earth’s surface 
of those vehicles in which they are located. In gen- 
eral, these devices obtain present position by inte- 
grating the vehicle’s velocity over the time it has 
been in motion. The position is usually indicated 
on “ present-position” dials as either the distance 
from a point of origin or as the latitude and longitude 
of the present position. GPI was designed to work 
with a radar mapping system. Although GPI can be 
used as a position indicator for any type of vehicle, 
it was intended primarily for use as a navigation and 
bombing computer for aircraft. 

In addition to the radar system, GPI requires in- 
put data from a true-airspeed meter and a compass. 
Other data such as altitude and bomb characteristics 
are set into the GPI computer by the operator. From 
this information, GPI computes and presents on dials 
the following quantities: 

1. Wind, expressed as north-south and east- west 
components. 

2. Altitude. 

3. Total distance traveled from some arbitrary 
origin (such as the base landing field), expressed as 
north-south and east-west components. 

4. Time which will be required to reach a prede- 
termined landmark or reference point, the particular 
landmark being chosen by making an electronic 
index (the intersection of a pair of cross hairs on the 
scope) coincide with the radar signal from the land- 
mark. 

5. Position relative to the landmark chosen as in 
(4), expressed as north-south (F) and east-west (X) 
components, i.e., fix data. 

6. Heading which must be steered in order to 
reach the point chosen as in (4) (shown on a pilot’s 
direction indicator [PDI]) . 

In addition to these outputs which the operator 
can observe, ground speed is obtained, but is fed 
directly to the integrators without being displayed 
on a dial. 

Fundamentally GPI is a navigational computer. 
However, bombing can also be done by navigating 


accurately to a release point and dropping bombs. 
Therefore, it is only necessary that ballistic data be 
entered into the computer for the device to become 
a GPI bombing computer. Sjnce the GPI bombing 
computer has been designed abcording to this philos- 
ophy, it presents a complete solution to the bombing 
problem. That is, it solves problems in navigation, 
identification, computation of the release point, and 
steering (see Chapter 6). 


9 . 1.1 Fix Determination 

The position of the aircraft is established with GPI 
by measuring (1) the difference in altitude between 
the aircraft and the reference point (P in Figure 1) 


HEADING 
OF AIRCRAFT 



h = altitude of aircraft 

R = ground range aircraft to reference point P 

p = slant range aircraft to reference point P 

dc = heading of aircraft 

6r = direction of reference point P 

d s = direction spinner points 

X = E-W component of R 

Y = N-S component of R 

Figure 1. GPI geometry of fix. 

which may also be the target, (2) the slant range to 
the reference, and (3) the direction of the landmark 
relative to the airplane. The information entering into 
the determination of fix is shown in Figure 1. First, 
the value for altitude ( h ) is set into the computer by 
the operator, who turns the altitude knob until a 
range mark coincides with the innermost radar signal 
appearing on the scope (the ground return or altitude 
signal). Secondly, the “fix” mileage X and Y dials 
are adjusted until the cross hairs appear on the 
radar signal reference, P . When this has been done 
the dials then show the altitude of the aircraft above 
the ground and its ground position with respect to 
the reference. (Ground position is read in rectangular 


RESTRICTED 


87 


88 


THE GPI BOMBING AND NAVIGATION COMPUTER 


coordinates on two dials, one corresponding to X 
miles in the east-west direction, the other to Y miles 
in the north-south direction.) From this point on, 
all of the GPI computations are made in a plane 
parallel to the earth’s surface, the altitude ( h ) factor 
being used only to convert slant-range radar data 
into GPI ground-range information. 

9.1.2 Rate Determination 

The ground rate (velocity) of the aircraft is de- 
termined by vector addition of the true airspeed and 
the wind (see Chapter 6). This vector addition is 
illustrated in Figure 2. 

The components of ground speed V gx and W gv are 
used to control the rate of change in magnitude of X 
and Y so that the electronic index will move on the 
radar scope with the same ground speed as the air- 

NORTH 



V„ = true airspeed vector 
V g = ground speed vector 
W = wind vector 
Wx = E-W component of wind 
Wy = N-S component of wind 
V (l x = E-W component of V a 
VaY = N-S component of V„ 

Vgx = E-W component of V 0 = V a x + Wx 
VgY = N-S component of V„ = F a r + Wy 

Figure 2. GPI geometry of rate. 

craft but in the opposite direction. As can now T be 
seen, if the index is placed over some radar landmark 
on the radar scope it will remain on that reference 
provided the values taken for the rates are correct 
and the coordinate systems used for rate and fix 
components are the same. If the aircraft should 
change heading, V gx and V gy would continue to repre- 
sent the coordinate sum of V a + W and the cross 
hairs would then remain coincident with the radar 
landmark. 

If, at a time when a wind W is blowing, zero wind 
is entered into the GPI, it is obvious that the index 


will not remain coincident with the reference. Rather 
V gx and V gy will represent components of the true 
airspeed V a only and the cross hairs will drift off 
from the landmark at the rate and in the direction of 
the wind W. In order that ground rates used by the 
GPI will be correct, the wind must be entered into 
the computer. This is done by a process known as 
memory-point tracking. It consists of making two 
fix determinations, comparing the ground distance 
traveled with the air distance (distance relative to the 
air mass) traveled between the two fixes, and divid- 
ing the difference in ground and air distance by the 
time necessary to accumulate that difference. The 
actual procedure is very simple as far as the operator 
is concerned. It consists of placing the electronic 
index coincident with the landmark signal on the 
radar scope, pushing a switch, waiting until the 
electronic mark drifts off the reference, and then re- 
setting it on the landmark. When this process is 
completed the wind computations are performed by 
the GPI and the rates which result should cause the 
marker to remain thereafter coincident with the 
radar signal. 

9.1.3 Course Determination 

The course of the aircraft is set by placing the 
electronic index on the point over which it is desired 
to fly. This can be done in either of two ways. The 
first method simply consists of placing the index in 
coincidence with a signal which can be seen on the 
radar scope. The second method performs the same 
function for points which do not appear on a radar 
scope. The latter method takes advantage of the 
GPI’s knowledge of present position and the op- 
erator’s ability to set the X and Y dials to desired 
future position numbers (see Section 9.4.2). Since the 
X and Y fix mileage dials define the position of the 
eross hairs, they will appear in the correct position on 
the radar scope whether a radar signal is there or not. 

The geometry of GPI course determination is illus- 
trated in Figure 3 w r here it can be seen that all factors 
are known from previous GPI computations except 
the time T. T represents the time necessary to reach 
point P at the present airspeed and with the present 
wind if a heading is taken so that the ground track of 
the aircraft passes directly through this point. 

Triangle POM is similar to the triangle P'O'M' of 
Figure 2, since their sides are proportional, and the 
proportionality factor is the time T. Since both R 
and V g are known, T can be determined if R is di- 


mm 


THE FUNDAMENTAL GEOMETRY OF THE GROUND POSITION INDICATOR [GPI] 


89 


vided by V g or, as in the computer described here, if 
R l is divided by V a . T is indicated on a meter and 
may be used by the operator to determine his time 
of arrival at destination. 


NORTH 



This diagram is similar to Figure 2 except that all 
rates have been multiplied by time T. 

T = time necessary to reach reference point P if a 
correct heading is maintained. T = R/V g = Rl/V 0 
V a T = R = ground range to reference point P 
WT = distance aircraft is blown in time T 
V a T = V„T — WT = Rz. = air range to reference 
point 

5 = drift angle 

The point M may be considered moving as time T 
changes; it will coincide with point P at time T equal 
zero; aircraft always heads toward M to reach point P. 

Figure 3. GPI course determination geometry. 

If T is determined in this manner, all the necessary 
geometrical data for Figure 3 are available in the 
computer. The computer then can establish the 
heading OM necessary if the ground track OP is to 
pass through the point P. This is true only if the 
cross hairs rest on point P. Having this knowledge, 
it is then possible to compare present heading as 
indicated by a compass, with desired heading in the 
computer, and indicate their difference on a meter. 
By flying so as to keep the meter indicating zero, the 
aircraft flies the desired course. 

9.1.4 Release and Course Determi- 
nation for Bombing 

The bombing problem can be divided into two 
parts, direct and offset. Direct bombing consists of 
releasing bombs so that they hit the target upon 
which the cross hairs have been placed. Offset bomb- 
ing consists of hitting a target located a known dis- 
tance and direction from the reference upon which 
the cross hairs are placed. 

The direct bombing problem involves the same 
course determination problems as given in Section 
9.1.3; however, the release point must be computed. 
The geometry used for direct bombing is given in 
Figure 4. 


From this figure it can be seen that the release 
points for the target Q are located on a circle of ra- 
dius | V a T f — T R | and whose center is upwind from the 
target by WT/. To reach this conclusion consider the 
target Q upon which it is desired to drop bombs. A 
vector AQ (equals LP) is drawn in the direction the 
wind is blowing, pointing at If this line represents 
wind times the time of fall of the bomb, it will repre- 
sent the distance the bomb is blown while falling. 
Next a vector LA (or PQ) is drawn in a direction 
opposite to the assumed heading of the aircraft. It is 
given a magnitude equal to the trail | T R | of the 
bomb. Trail represents the distance the bomb falls 
short of its objective because of the decrease in hori- 
zontal speed after release. The decrease in speed is 
caused by the resistance of the air to the motion of 
the bomb. Now draw the vector RL through point L, 
which is determined from PQ and PL, in the direction 



Tf = time of fall of the bombs 

Other symbols the same as for Figures 2 and 3. 
Figure 4. GPI indirect bombing geometry. 

of the heading of the aircraft and give it a magnitude 
| V a Tf |. RL represents the distance the bomb would 
have fallen if there had been no air resistance or 
wind. Now it can be seen that a bomb released at R 
will hit Q. Furthermore, it can be shown that for a 
particular wind, time of fall, and trail, any heading 
towards point A will result in bombs on the target if 
release occurs at the bomb release circle. 

It now becomes obvious that this geometry is only 
a special case of that discussed under course determi- 
nation, providing trail is subtracted from \ a T and T 
is replaced by T f . This being so, steering can be 
achieved in the same manner as previously, bombs 
being released when T becomes equal to T f . It is also 
possible to indicate, on the time-to-go meter, the 


RESTRICTED # 


90 


THE GPI BOMBING AND NAVIGATION COMPUTER 


difference between T and T f which is the time-to-go 
to release. Time of fall and trail are factors which are 
set into the computer from empirical data compiled 
in standard bombing tables. 

The offset bombing problem is simply a variation 
of direct bombing as can be seen in Figure 5. 

In the case of offset bombing the distance from air- 



Xo — E-W component of offset distance 

Other symbols same as for previous figures. 

Figure 5. GPI offset bombing geometry. 

craft to target can be considered as made up of two 
components. The first is the distance from aircraft to 
a radar reference point. The second is the distance 
from reference point to target. The distance from 
reference point to target is a geographical factor 
which can be determined from maps and entered into 
the computer as components, X 0 and Y 0 . The dis- 
tance from aircraft to reference is obtained as always 
by placing the cross hairs on the point and using the 
computer to determine the X and Y components of 
the distance. Having the components of offset and 
fix, the computer can add them and determine the 
distance from aircraft to target. Once this has been 
established the entire offset bombing problem be- 
comes a direct bombing problem and is handled by 
the computer in the same manner as previously dis- 
cussed. 

9.2 THE GPI COMPUTER MECHANISM 

In the following step-by-step discussion of the 
GPI computer, frequent reference will be made to 
Figure 6. This figure has been divided into three 
sections containing (A) the rate mechanisms, (B) the 
fix mechanisms, and (C) the course and bomb-release 
mechanisms. In this figure dashed lines represent 


mechanical linkages while solid lines are electric 
connections. 

9.2.1 Fix Computation 

Fix measurements are made with the function 
switch (marked N-C-B for navigate-course-bomb) in 
the navigate ( N ) position. Referring to Figure 6, 
part B, it is seen that the precision helipot (multi- 
revolution potentiometer) (1) has an a-c voltage 
across it supplied by transformer (2), the center tap 
of which is grounded. The voltage output of the heli- 
pot will vary from full voltage at one end, to zero at 
the center, to full voltage at the other end, as the 
potentiometer arm (3) sweeps across the helipot. It 
should also be noted that there is a phase reversal of 
the voltage as the potentiometer arm passes over the 
center of the helipot wdiere the voltage passes through 
zero. The shaft of the helipot (1) has been fitted with 
a knob (4) and a dial (5). If a scale factor of some 
number of nautical miles per volt output is decided 
upon, the dial (5) can be given markings which indi- 
cate the helipot arm position in nautical miles. Since 
the helipot is linear, the dial markings will be uni- 
form. At this point, it is observed that an adjustment 
of dial (5) to any number will result in the presenta- 
tion of a proportional voltage X to the driver (6). 
The dial calibration for the computer is such that the 
voltage from ground to one end of the helipot repre- 
sents 20 nautical miles. The driver (6) is an imped- 
ance-matching device with an input-output voltage 
ratio of 1/1. The input to the other driver (7) is de- 
rived in the same fashion as that described for driver 
(6). The voltage X L from driver (6) represents at all 
times the algebraic sum of the E-W components of 
distance to be introduced to the Arma resolver (8). 
In this instance, the voltage Xl represents only the 
distance X shown in Figure 1. The two a-c voltages 
X l and Y l are the inputs to the Arma resolver (8) . 

The Arma resolver is a device used to perform vec- 
tor addition of a-c voltages. It has two stator wind- 
ings displaced 90 mechanical degrees from one an- 
other and two rotor windings similarly displaced. If 
first the two stator windings are excited by voltages 
Xl and Y L respectively, and if next the rotor is 
turned until the voltage output of one of its coils is 
zero then the output of the second rotor coil will be 
the resultant of the two input voltages; R L = 

( Xi + Yiy. Also, the position of the rotor shaft 
will then represent the angle whose tangent is 
X l /Y l . In practice, the shaft of resolver (8) is turned 


.ESTRICTED ' 


THE GPI COMPUTER MECHANISM 


91 


by a motor (9) under control of servo-amplifier (10) 
until the voltage output 0 of the resolver is zero. 
Under this condition, the other resolver output volt- 
age Rl is equal to the square root of the sum of the 
squares of X L and Y l and represents the distance R 
shown in Figure 1. 

The voltage Rl is applied through switches (12) 
and (13) to the altitude triangle solver (14). The tri- 
angle solver also has applied to it a voltage in quad- 
rature with R l controllable by the altitude knob (15). 
The potentiometer turned by knob (15) has a scale 
factor (miles per volt) proportional to the rest of the 
system, but for convenience the altitude dial (16) 
markings are in feet instead of miles. The output 
(p) of the triangle solver is the resultant of the volt- 
age R l and the altitude voltage (h) and is propor- 
tional to slant range. The voltage (p) enters a range 
mark generator (17) to produce a pip which in turn 
enters the mixer (18) and thence to the PPI (19) 
appearing finally as a range circle (20). 

The azimuth mark portion (21) of the index comes 
from the addition of three selsyn voltages. The first 
voltage, d c , represents the heading of the aircraft. The 
second voltage, d s , represents the direction the an- 
tenna (scanner) points relative to the aircraft’s head- 
ing. The third voltage, d R , represents the Arma re- 
solver position. Since d s continually changes as the 
radar scanner turns, the azimuth mark generator is 
designed to produce a mark whenever the sum of 0 C 
and d s equals 0 R . This is done by causing a PPI sweep 
to be brightened as the scanner passes through the 
position where 6 S = d R — 6 C (see Figure 1). 

The intersection of the range mark (20) and the 
azimuth mark (21) constitutes an index, the ground- 
range components of which are the mileages shown 
on the two fix dials (5) and (51), providing the 
proper altitude has been entered into the triangle 
solver (14). The altitude is determined by first 
grounding the switch (13), which then gives com- 
plete position control of the range mark to the alti- 
tude voltage (h), and by turning knob (15) until the 
range mark (20) coincides with the innermost signal 
on the PPI. Since this innermost signal is the ground 
return, the altitude knob will have been turned until 
the dial (16) reads the true radar altitude and the 
voltage into the triangle solver (14) will be correct. 

9.2.2 Rate Computation 

Referring to Figure 6A, it is seen that a d-c voltage, 
V a , taken from the true airspeed meter (22) has been 


resolved into components by means of a sine potenti- 
ometer (23) which turns with the shaft of the compass 
(24). If the “sinepot” (23) is properly adjusted, its 
two voltage outputs will be V a sin 0 C or V ax , and V a 
cos d c or V ay, respectively, where d c is the heading of 
the aircraft. Since the two components are treated 
similarly, only the E-W component will be dis- 
cussed here. The side of the servo-amplifier (25) con- 
nected directly to the sinepot will be considered as a 
reference voltage level. Continuing from this point 
through the sinepot (23), the generator (27), the 
switch (26), and the second generator (28) to the 
opposite side of the servo-amplifier (25), it is found 
that, if both generators are stationary and the 
switch (26) is in the “reset” position (72), the voltage 
across the servo-amplifier (25) is V a sin d c or V ax . 
However, since the servo-amplifier acts to reduce 
any voltage across its terminals to zero, it starts the 
motor (29) by means of control (30). The motor 
speed will be of such a magnitude and direction that 
the generator (28) will produca a d-c voltage equal 
in magnitude to V a sin d c and opposite in polarity to 
the voltage from the sine potentiometer. The servo- 
amplifier (25) then will have virtually zero voltage 
across it and, if generator (28) has a linear relation- 
ship between voltage output and shaft revolutions per 
minute, the shaft (31) will be revolving at a rate 
proportional to the E-W component of airspeed (V a 
sin d c ). If this shaft (31) is connected by means of 
clutch (32) to the fix-dial shaft (33) (Figure 6B), the 
cross hairs can be caused to move in an E-W direction 
at a rate proportional to the E-W component of air- 
speed. In a similar manner, the N-S velocity com- 
ponent can be connected to the N-S fix shaft so that 
the radar index is caused to move in a N-S direction 
opposite to the N-S air motion of the aircraft. 

The motion which has been imparted to the radar 
index so far is proportional to airspeed. In order that 
this movement be proportional to ground speed, the 
voltage equivalent of wind must be added to each of 
the airspeed component voltages. This can be done 
by moving potentiometer arm (34) so that the servo- 
amplifier (25) is biased in a manner causing the 
generator (28) output to be equal to the sum of V ax 
and W x (Figure 6A). Conceivably, this adjustment 
could be made by a trial and error process of chang- 
ing the potentiometer arm (34) position until the 
radar index would remain in coincidence with any 
radar reference on the PPI. However, since this 
would be a tedious process, the memory-point track- 
ing procedure was devised. 



92 


THE GPI BOMBING AND NAVIGATION COMPUTER 




PART A 


Figure 6A. GPI functional diagram. 


“•RESTRICTED 



THE GPI COMPUTER MECHANISM 


93 



Figure 6B, C. GPI functional diagram. 


1 


.ESTRK’TKD 






94 


THE GPI BOMBING AND NAVIGATION COMPUTER 


When the knob (35) is turned, generator (27) pro- 
duces a voltage which is added in series with the 
voltage Vax . This results in an increase or decrease in 
the speed of motor (29) for the length of time that 
generator (27) is in motion. This, in turn, causes the 
radar index to increase or decrease speed tem- 
porarily and effectively results in a small position 
change of the index. Since d-c voltage is proportional 
to velocity in this circuit, the product of d-c voltage 
and the time it is acting will be proportional to a 
distance or displacement. The control (35) is used as 
a vernier for displacement of the cross hairs, while 
the fix knob (4) is a coarse adjustment. 

Now, to determine wind, the radar index must 
first be placed in coincidence with a fixed signal ap- 
pearing on the PPI scope. This adjustment may be 
made with either or both of knobs (4) and (35). 
Secondly, the switch (26) is thrown from reset (R) to 
track (T) position; this also closes clutches (36), (37), 
and (38) and starts the constant-speed motor (39). 
Assuming that the effect of the E-W wind potentiom- 
eter (40) was initially that of zero wind, the cross 
hairs would drift away from the radar signal at a rate 
proportional to the X component of true wind. The 
constant-speed motor (39) moves the potentiometer 
arm (41) uniformly across potentiometer (42). Be- 
cause of its constant speed, the position of the arm 
(41) can be considered a measure of the time ( t ) 
since the motor started. 

It can be shown that, if the cross hairs are reset 
on the fixed radar reference at the end of time (fa) by 
means of knob (35), the shaft (43) will have been 
turned an amount proportional to the X component 
of wind. This follows since, after time fa, the dis- 
placement error caused by an incorrect rate setting 
will be proportional to fa. Since the fraction of the 
generator voltage which is obtained by the moving 
potentiometer arm (41) will be proportional to time, 
we have a means whereby the correct relationship 
between the displacement and the rate correction can 
be determined. A more detailed discussion of this is 
given in the following paragraphs. 

In the memory-point tracking operation, the rotation of 
knob (35) controls the shaft (43) and the setting of arm (34) 
of the d-c wind potentiometer. If the instantaneous position 
of this shaft is designated W x , its rotation will then result in 
an output C ( dw x /dt ) from the generator (27) and C ( dw x /dt ) • 
( t/T i ) at the potentiometer arm (41) controlled by the constant- 
speed motor (39). Here C is a constant, t the time elapsed 
since the clutching-in of the constant-speed motor (39) at the 
beginning of the memory-point tracking procedure, and T t the 
total time necessary for the arm (41) to traverse the entire 


range of potentiometer (42). In the servo-loop, such an output 
represents a rate — e.g. V ax . 

The drift of the cross hairs from the target since the start 
of the procedure results in a displacement (in the X coordinate) 
of 



in which W x is the true component of wind as distinguished 
from the instantaneous setting w x = w x ( t ), and the last term 
of the integral represents, of course, the effect of the generator 
output as modified by the setting of potentiometer (42). If, 
now, the cross hairs are again made coincident with the 
target at a time fa % T t , the value of this integral becomes 
zero for t = fa. 

Accordingly, after integration of the last term by parts 
there results: 


[V (W'-V' + C'g) 


= 0 . ( 1 ) 


If C is made equal to T t , this becomes 
W x -fa = w x (fa) • fa. 

Thus the resulting value w x (fa) is the true wind component, 
W x , and the cross hairs should thereafter track the target 
automatically. 48 * 1 


Since the rotation of shaft (43) is proportional to 
E-W component of wind , it is only necessary to con- 
nect it to the arm (34) of the d-c wind potentiometer, 
which, if properly calibrated, will cause the motor 
(29) to run at the E-W component of ground speed. 
A calibrated dial (59) may be connected to this shaft 
to indicate wind velocity. 

Since the clutch (37) is connected only during the 
time a wind determination is being made, the value 
for wind remains constant once it is determined, and 
knob (35) can again be used for vernier displacement 
adjustments. The spring (44) is used to return arm 
(41) to zero, when the switch (26) is in position R, so 
that a new wind determination will be possible. 

A present position dial (45) is connected to shaft 
(31) through a slip clutch (46). This dial can be set 
to read zero for any position by means of knob (47) 
and then will continuously indicate the E-W distance 
in miles to that point, since it is connected to a shaft 
rotating at a speed proportional to the aircraft’s 
ground speed. 


9.2.3 Course Computation 

By means of a switch and some of the components 
already described for the determination of fix, it is 
relatively simple to determine the heading the air- 
craft should take in order that its true ground track 
will pass over the ground point designated on the 



THE CPI COMPUTER MECHANISM 


95 


radar scope by the cross hairs. The time which will 
elapse before the vehicle reaches the designated point 
can also be found. The means by which these quanti- 
ties are obtained are described in the following 
paragraphs, which treat first the method of obtaining 
the time-to-go, T. The geometry to be solved is 
shown in Figure 3. The switches marked N-C-B in 
Figure 6 will be in the C (course) position for this 
portion of the discussion. 

Since the computer used for course computation is 
of the type which solves geometric problems by 
means of a servo-loop, it is simplest to make an 
assumption regarding some point in the loop and 
later prove that this assumption is correct. Referring 
to Figure 6C, it is assumed that the a-c voltage input 
(T) to transformer (48) is proportional to the time 
required for the aircraft to proceed from point 0 to 
point P in Figure 3. The point P is the one designated 
by the cross hairs. The time T is obtained through 
the use of the circuit loop containing the transformer 

(48) and the airspeed division circuit (52). The out- 
put of the transformer (48) is a voltage of magnitude 
e 0 , which will be found to be proportional to the 
time-to-go, T. This voltage excites the potentiometer 

(49) and (in the E-W channel) results in a voltage at 
the arm (50) which is a constant ( k ) times e 0 W x . 
Since the switches (60) and (61) are now in the 
Course position (C), this voltage is subtracted from 
that obtained from the potentiometer (1), which 
gave us a measure of the distance X (Section 9.2.1). 
Accordingly, the input, via driver (6), to the Arma 
resolver (8) is (. X — ke 0 W x ) . By combining this input 
with the analogous input for the N-S channel, the 
output of the resolver will then represent 

[X + Y - ke 0 (W x + W„)], or [_V g T - ke 0 W]. 

By the action of switch (12), which is also in position 
(C), the resolver output is connected to the airspeed 
division circuit (52). Since the function of this cir- 
cuit is to divide by the airspeed, V a , and because its 
output is the source of transformer voltage, e 0 , we 
are led to the following relation for the circuit loop 
under consideration: 


V„T-ke 0 W 


= ke 0 or 


— • — W 

kc 0 


= I V a .(2) 


The inputs, V a , V g , and W are, however, connected 
by the vector relation V g — W = V a ; accordingly we 
are led to identify ke Q with T. For reasonable values 
of V a and W, that is, such that | V a | > | W | , there 


is only one positive value of e oy namely, e 0 = T/k 
which will satisfy the relation 


I V. | = 


T 

— • V, - W 


subject to the condition V ff — W = V a . In the fol- 
lowing paragraph it will be sejen how the division by 
airspeed is achieved, but, granting that this has been 
done, we see that jFe output of the unit (52) is a 
voltage which will serve as a measure of the time- 
to-go, T. 

In the airspeed division circuit (52) the following 
relation holds: 


e ( 


ei 


R 0 

Ro + r 


(3) 


assuming the impedance fed by e 0 is large. In this 
equation r is equal to K(V a — V ao ), e 0 is the output 
voltage, and e\ is the input voltage to the resistor 
network. V ao is a constant which will be referred to 
as the lower limit of airspeed, and changes in the 
magnitude of r are proportional to changes in V a . 
Since r = K(V a — V ao ), equation (3) may be writ- 
ten as 


R 0 


e ° Cl R 0 + KV a - KVa 
If R 0 is adjusted to equal KV ao , then 

X- Vao F ao 

e ° = ei Kv a = e ’ y: 


(4) 


and the desired variation of e 0 with V a is obtained. 
Suppose next the input voltage is adjusted by moving 
the potentiometer arm (53) until 


Then 



(5) 

(6) 


This represents a voltage which, through the use of 
the desired proportionality constant k, serves as a 
measure of the time-to-go, T. 

The voltage e 0 which is proportional to T can be 
introduced to the time-to-go circuit (54) and thence 
to the time-to-go meter (55), which indicates the 
length of time before the aircraft arrives at the point 
indicated by the cross hairs. Likewise, since the in- 
puts to the Arma resolver (8) are now proportional 


RESTRICTED 


96 


THE GPI BOMBING AND NAVIGATION COMPUTER 


to the components of air range, X L and Y l, the 
shaft position of the resolver will represent the direc- 
tion of vector OM (Figure 3), and an indication of 
the desired heading can be obtained. This is done by 
comparing the information of the compass selsyn 
( 0 C ) and the resolver selsyn (Or) when switch (55) is in 
the course (C) position. The difference between these 
selsyn outputs is introduced to the phase detector 
(57), and thence to the PDI meter (58), which gives 
the direction the aircraft should turn to be on the 
proper heading. Figure 3 shows that this heading 
will be up-wind from the desired ground track by an 
angle equal to the drift angle (6). Once the correct 
wind information has been found by the GPI, the 
computer will solve the course geometry auto- 
matically for all directions of approach to the 
point P. 

9.2.4 Release and Course Compu- 
tation for Bombing 

The mechanism by which bomb release and course 
computation is achieved is very much the same as 
described for course computation in the preceding 
section. The geometry which will be discussed herein 
will be that for offset bombing (Figure 5). To make 
this discussion applicable to the direct bombing 
figure, simply consider the values for X Q and Y 0 
(offset distance) to be zero. The switch marked 
N-C-B of Figure 6 will be in the B (bomb) position for 
this portion of the discussion. 

As in the previous section, the input to trans- 
former (48) will be considered to be T, the time to 
proceed from present position 0 to future position P 
of the aircraft. However, here, point P will be chosen 
to be the position of the aircraft at the time the 
bombs reach the ground. As previously showm the 
output of potentiometer (49) is W X T. Since the 
switches (60) and (61) are now in the bomb (B) 
position, the displacement of arm (3) of potentiom- 
eter (1), and of arm (62) of potentiometer (63) will 
be added algebraically to that of the arm (50) of 
potentiometer (49). The resultant voltage output, 
X L , will be equal to (X + X 0 + W X T). Since a simi- 
lar procedure takes place in the N-S branch of the 
computer, the input to the Arma resolver (8) will 
again consist of Xl and Y l . This time, however, the 
resolver input voltages are X L = X + X 0 + W X T 
and Y L = Y + Y 0 -\-W v T. The Arma resolver (8) 
will act in the same fashion as previously described 
and the voltage output R L will be the resultant of X L 


and Y L . Referring to Figure 5, it can be seen that the 
vector Rl is the equivalent of (V a T — Trail). If then 
the voltage R L is taken through switch (12) to the 
trail circuit (64), where the voltage equivalent of the 
trail distance is added, the output will be V a T. Then, 
as previously, the voltage V a T is introduced into the 
airspeed dividing circuit (52), where it is divided by 
airspeed V a , and the output voltage e Q is equivalent 
to time T. The calibration of the airspeed dividing 
circuit (52) is the same as discussed in the previous 
section. 

The voltage e Q , which is proportional to time T, 
can now be introduced into the time-of-fall circuit 
(54), where it is compared with the known time-of- 
fall, T f , of the bomb. The difference between T and 
T f is then indicated on the time-to-go meter as the 
time-to-release-point. The voltage differential can 
also be used to actuate an automatic bomb-release 
circuit. The heading necessary to fly the ground 
track of Figure 5 is computed in the same manner as 
discussed in the part on course determination. 

9.3 REQUIREMENTS WHICH THE USE 
OF GPI PLACES ON ASSOCIATED 
EQUIPMENT 

In order that accurate computations can be made, 
it is necessary that the data used for computing be 
correct. How big an error can be tolerated in the 
data is entirely dependent upon the desired accuracy 
ol the answer. In the case of GPI bombing it is im- 
mediately apparent that the answer should be correct 
to within a few feet. However, for navigation an 
error ten or more times larger could usually be toler- 
ated. As a result, the requirements placed on the 
data-producing mechanisms associated with GPI are 
much more stringent for bombing than for naviga- 
tion. However, since the distances and times involved 
on a bombing run are much less than those for navi- 
gation, the difference in requirements for the two are 
not nearly so great as it at first appears. 

The accuracy requirements of the several com- 
ponents associated with GPI vary widely with the 
use to which GPI is to be put. In the following dis- 
cussion the effects of errors in the various associated 
components upon the GPI will be considered. 

9.3.1 Compass Requirements 

The most important single factor affecting the per- 
formance of GPI is the compass accuracy. Since the 




RE&TIUCTI-: 


REQUIREMENTS WHICH THE USE OF GPI PLACES ON ASSOCIATED EQUIPMENT 


97 


GPI integrates the aircraft’s velocity continuously 
along components of the aircraft’s heading, the ve- 
locity must be resolved correctly regardless of ma- 
neuvers of the aircraft. Similarly, the fix and steering 
data are resolved into components dependent upon 
the compass; therefore their accuracy also is a direct 
function of the compass accuracy. 

It is possible for GPI results to be affected by any 
of three different types of compass errors. The first 
of these is a random fluctuation about the indicated 
heading, which occurs regardless of the aircraft’s ma- 
neuvers. The second type of error is constant for a 
particular heading and usually results from errors in 
alignment, calibration, or deviation. Transient errors 
or temporary errors introduced by maneuvers of the 
aircraft are the third source. 

Random errors affect the ability of GPI to navi- 
gate or bomb mainly by virtue of the vector rate 
errors which they introduce when a wind determina- 
tion is made. In this case, the only part of the random 
error that matters is the change in error between the 
time of the first fix and the time of the second fix. 
The significance of these errors is illustrated by the 
figures shown in Table 1. In addition, random compass 
error at the time of the second fix will introduce a 
small fix error; but this is insignificant compared to 
the rate error. 


Table 1 . GPI rate errors caused by random compass 
errors. 


Change in compass 
error between fixes 
(degrees) 

Time between 
fixes 

(minutes) 

Fix 

distance 

(miles) 

GPI rate 
error 
(mph) 

0.5 

1.0 

10 

5.2 

1.0 

1.0 

10 

10.4 

0.5 

2.0 

10 

2.6 

1.0 

2.0 

10 

5.2 


Constant errors affect the GPI in three ways; first 
by presenting an erroneous indication of distance 
traveled, second by causing an erroneous fix to be 
taken, and third by causing erroneous rates as a re- 


Table 2. GPI error in indication of distance traveled 
caused by constant compass errors. 


Compass error 
(degrees) 

GPI error 

(per cent of distance traveled) 

1 

1.74 

2 

3.49 

5 

8.72 


suit of the fix errors introduced. The first of these is 
the direct result of rates being resolved into com- 


ponents about the wrong heading. This results in a 
cumulative position error. Figures for this are shown 
in Table 2. Fix error is caused by improper resolving 
of the fix components, the significance of which is 
illustrated in Table 3. 


Table 3. GPI fix error due to constant compass error. 


Compass error 

GPI error 

(degrees) 

(per cent of fix distance) 

1 

1.74 

2 

3.49 

5 

8.72 


Even though the compass error may be constant, 
it is possible that its existence will introduce a vector 
rate error. The reason for this is that the rates are 
all resolved about an erroneous heading and so their 
direction is wrong. This will not disturb the GPI 
operation unless an attempt is made to refer to the 
earth. Then, for instance, the direction of the wind 
vector would be in error — in fact, the solution of 
the entire problem would be in a coordinate system 
rotated with respect to the assumed N-S, E-W axes. 
The error is introduced into the GPI because fixes are 
taken at different ranges from the radar reference. 
With a constant compass error this results in differ- 
ent fix errors. Since rate errors are a function of the 
difference in fix errors, a rate error will result from 
this constant compass error. This is illustrated in 
Table 4. 


Table 4. GPI rate error due to constant compass error. 


Compass 

error 

(degrees) 

Distance of 
fix one 
(miles) 

Distance of 
fix two 
(miles) 

Time be- 
tween fixes 
(minutes) 

GPI rate 

error 

(mph) 

1 

10 

7 

1 

3 

1 

10 

4 

2 

3 

2 

10 

7 

1 

6 

2 

10 

4 

2 

6 


The effects of transient compass errors on GPI can 
appear in three different w'ays. The first of these is 
an erroneous indication by GPI of distance traveled . 
Each time a transient error occurs the rates will be 
resolved about the wrong heading and position error 
will accumulate. The size of the GPI error will de- 
pend entirely upon the magnitude of the transient 
and the time for which it persists, as well as on the 
velocity of the aircraft. Transient errors usually arise 
as a result of turns made by an aircraft. Typical 
figures illustrating these errors are given in Table 5. 

The second effect of the transient compass error 



98 


THE GPI BOMBING AND NAVIGATION COMPUTER 


Table 5. Position error accumulated per turn because 
of transient compass errors. 


Compass error 
(averaged over 
the time that 
the error 
persists) 
(degrees) 

Time that 
the error 
persists 
(minutes) 

Typical 

turning 

time 

(minutes) 

Aircraft 

velocity 

(mph) 

Position error 
(accumulated 
for each 
turn made) 
(miles) 

1 

1 

0.5 

180 

0.05 

1 

2 

1 

180 

0.10 

1 

1 

0.5 

240 

0.07 

1 

2 

1 

240 

0.14 

5 

1 

0.5 

180 

0.25 

5 

2 

1 

180 

0.50 

5 

1 

0.5 

240 

0.35 

5 

2 

1 

240 

0.70 


is to cause fix errors. The fix error is entirely de- 
pendent upon the magnitude of the compass error 
and is the same as shown in Table 3. 

The third effect of transient errors is to cause rate 
errors the same as those caused by random errors 
(Table 1). In this case, however, the rate errors 
brought about by the erroneous fixes would be 
larger, because changes in transient errors are usually 
of greater magnitude than the customary random 
errors. Further illustrations of these errors are given 
in Table 6. 


Table 6. GPI rate errors caused by transient compass 
errors. 


Change in 




compass error 

Time be- 

Fix 

GPI rate 

between fixes 

tween fixes 

distance 

error 

(degrees) 

(minutes) 

(miles) 

(mph) 

1 

1.0 

10 

10 

5 

1.0 

10 

52 

10 

1.0 

10 

105 

1 

2.0 

10 

5 

5 

2.0 

10 

26 

10 

2.0 

10 

52 


In summary, it can be stated that fix and integra- 
tion errors are the two major errors introduced di- 
rectly by the compass. Rate errors in wind deter- 
mination are brought in by a change in fix error re- 
gardless of the source of error, but a major part of 
this also comes from the compass. 

At the present time, a number of compasses exist 
having varying degrees of accuracy, but all are some- 
what faulty in construction. The Gyrosyn built by 
Sperry Gyroscope Company seems the most satis- 
factory of those investigated; furthermore it has the 
advantage that its basic design permits simple modi- 
fications which greatly increase its accuracy. All 


compasses for aircraft as of 1945 are subject to 
turning errors which may be of the order of several 
degrees. 21 

9.3.2 Airspeed Meter Requirements 

GPI requires a voltage proportional to true air- 
speed. True airspeed, as distinguished from the 
commonly used indicated airspeed, is a measure of 
the true velocity of the aircraft relative to the air 
mass in which it flies. Indicated airspeed is simply 
an indication of the relative pressure exerted by the 
air mass in the direction opposite to the motion of 
the aircraft, and as such depends upon the density 
of the air which varies with altitude. A number of 
commercial true airspeed meters exist as of 1945 
which fulfill the voltage requirement; some require 
slight modification. 

The accuracy requirement placed upon the air- 
speed meter is again a matter of judgment which de- 
pends upon the bombing and navigational needs. Of 
course, if it is permissible to “determine wind” each 
time the airspeed or heading changes, almost any air- 
speed meter will do, since computed fictitious winds 
will compensate for the airspeed errors. However, if 
any degree of freedom of operation is desired and a 
correct value for wind is wanted, the airspeed require- 
ments are very stringent. 

Airspeed errors are also of three types — random, 
transient, and constant. The random errors do not 
affect GPI operation unless the magnitude of vari- 
ation is extreme. Since most meters are satisfactory 
in this respect, random errors will not be discussed 
further except to say that they cause instantaneous 
position errors which are averaged out in the total 
mileage indication. 

Transient airspeed errors usually occur in the form 
of a lag in response to changes of aircraft speed or 
altitude. This lag can cause either an integration 
error, or if it occurs during a wind determination, a 
rate error. Since in most airspeed meters the lag is of 
small magnitude and time duration and, also, since 
it is common for aircraft to fly for long periods at a 
relatively constant speed, the integration errors from 
this source are negligible. Moreover, if necessary, a 
GPI operator can avoid any error from this source in 
wind determination, by determining wind only when 
no radical changes in airspeed are being made. 

The principal errors in present day airspeed meters 
are the constant errors. These are usually caused by 


faulty calibration or improper installation. The effect 



REQUIREMENTS WHICH THE USE OF GPI PLACES ON ASSOCIATED EQUIPMENT 


99 


of a constant airspeed error is to cause the GPI to 
compute a wind error of equal magnitude and oppo- 
site direction. An error of this type will not cause any 
error in the indication of distance traveled as long as 
the heading on which the wind was determined is 
maintained. However, if a change in course is made, 
both an incorrect heading and rate will be indicated. 
As a result of the heading and the rate errors, the 
position indicated on the GPI dials will be in error 
by an amount e which can be expressed as 

e = 0.33(aF„)(<j(sin|) (7) 

where € = position error in miles, 

8V a = constant airspeed error in mph, 

A = angular measurement of change in head- 
ing, 

t e = time in minutes elapsed since turn was 
made. 

Table 7 gives some typical values for errors that 
might be expected from this source. 


Table 7. GPI position error in miles caused by con- 
stant airspeed error. 


Change of heading 
(degrees) 

True airspeed error 

1 mph 5 mph 10 mph 

A. 10 minutes after 

the aircraft 

has been 

turned 

0 

0.00 

0.00 

0.00 

5 

0.01 

0.07 

0.14 

10 

0.03 

0.14 

0.29 

20 

0.06 

0.29 

0.57 

30 

0.08 

0.43 

0.85 

90 

0.24 

1.17 

2.36 

180 

0.33 

1.65 

3.30 

B. 30 minutes after 

the aircraft 

has been 

turned 

0 

0.00 

0.00 

0.00 

5 

0.04 

0.22 

0.43 

10 

0.09 

0.43 

0.86 

20 

0.17 

0.86 

1.72 

30 

0.25 

1.28 

2.55 

90 

0.70 

3.50 

7.00 

180 

0.99 

4.95 

9.90 


In general, the true airspeed information available 
for GPI operation is sufficiently good when compared 
with other data such as the compass data. However, 
care should be taken when an airspeed meter is in- 
stalled, for even the best meter performs badly unless 
it receives proper pressure and temperature informa- 
tion. If considerable care is taken during installation 
and calibration, it will be found that the effect on 
GPI of airspeed errors will be negligible. 


9.3.3 Radar Requirements 

GPI requires that the radar system with which it 
is used provide a synchronizing pulse (trigger) as 
well as scanner azimuth data. The trigger is usually 
derived from the modulator output pulse. Azimuth 
data is received from a selsyn unit rotated by the 
radar antenna assembly. In turn (see Section 9.2.1), 
GPI provides an azimuth mark and a range mark as 
an index on the radar scope. 

The previous discussion (Section 9.2.2) has shown 
how GPI depends upon fix information. The compu- 
tation of fix determines present position relative to 
a reference point. Because of these two facts, any- 
thing which contributes to the accuracy of fix de- 
termination is important to the GPI performance. 

The resolving power of a radar system is one factor 
which determines how accurately fixes can be made. 
Moreover, as was 'shown in Chapter 7, resolution de- 
pends upon the transmitted frequency, pulse dura- 
tion, beamwidth, size of the PPI indicator spot, PPI 
sweep speed, and the persistence of the tube. As far 
as the present discussion is concerned, most radar 
systems have sufficient resolution to make extremely 
good navigational devices. 52a On the other hand, the 
beamwidth, spot size, sweep speed, and tube per- 
sistence, which contribute to GPI fix accuracy, leave 
something to be desired. 

Since the accuracy of GPI fix is dependent upon 
how well a reference signal is seen and upon the ac- 
curacy with which an index can be brought into co- 
incidence with that signal, it is logical to examine 
this process. It is assumed, for the present, that the 
method of setting cross hairs on the radar reference is 
to make the azimuth mark bisect the reference and to 
set the trailing edge of the range mark just tangent 
to the leading edge of the reference. Accuracy of the 
range adjustment is largely dependent upon the spot 
size and sweep speed of the PPI tube. The accuracy 
of azimuth adjustment depends upon the width of 
the radar beam, and the range and size of the target. 
Both of these factors are affected by the persistence 
of the radar scope if the aircraft is traveling at high 
speed or is rolling and pitching with unstabilized 
antenna. 

The sweep speed which should be employed on the 
radar set used with GPI depends once more upon the 
needed navigational accuracy. If the trailing edge of 
a range mark is considered to fall instantaneously, 
then the apparent point at which it falls as it appears 
on the PPI will be one-half the spot size of the tube 


100 


THE GPI BOMBING AND NAVIGATION COMPUTER 


beyond the true point. Likewise, if a signal rises 
instantaneously, it will appear to start one-half spot 
size ahead of its true position. Thus, if the trailing 
edge of a range mark and the leading edge of a signal 
were made tangential as they appear on a PPI, a gap 
would exist the size of one spot between the true 
edges. Since the spot size of a tube is constant re- 
gardless of sweep speed, the distance in miles that is 
represented in a spot size will change with sweep 
speed. If a signal does not rise rapidly, the previous 
statements are still true and, in addition, the true 
leading edge and trailing edge will also change as a 
function of radar receiver gain and PPI brilliance ad- 
justment. Assuming a spot size of 0.5 mm, Table 8 


Table 8. Range accuracy as a function of sweep 
speed (assuming PPI spot size is 0.5 mm). 


Sweep speed 
(miles per inch) 

Range accuracy 
(miles) 

8 

0.16 

6 

0.12 

4 

0.08 

2 

0.04 

1 

0.02 


shows the range accuracy which might be achieved if 
it is assumed that the gap between range mark and 
signal is one spot size. 

From Table 8, it is apparent that the absolute 
range of a target can be calibrated conveniently for 
only one particular sweep speed, since the spot size 
causes different apparent ranges on different sweeps. 
Also an erroneous rate of 2.4 mph would be measured 
if the sweep speed were changed from 4 miles per inch 
to 2 miles per inch during the process of measuring 
wind with GPI, and if the time between fixes were 
one minute. This would be caused by the difference 
in apparent range between the first and second fix 
of the wind measurement. For these reasons, it is 
recommended that one standard sweep speed be used 
when making range measurements. It is apparent 
that, if the sweep speed is to remain constant, the 
maximum range at which a fix can be taken will be 
the product of the sweep speed (in miles per inch) 
and the radius of PPI scope (in inches) unless special 
provision is made. It is suggested that the most de- 
sirable method for avoiding a limitation in range is 
to use a delayed sweep, preferably of the offset-center 
type. This permits the use of a fast sweep with low 
distortion at ranges much greater than otherwise. 

Experimental tests performed by the authors have 
shown that a range mark can be adjusted repeatedly 


to the same point on the face of the PPI tube within 
±0.01 in. using the equipment discussed here. This 
figure is sometimes called setability or resetability . It 
should be remembered that this figure for setability 
is necessarily true only for the method employing 
tangential comparison of two marks. 

The technique of bisecting a radar reference with 
an electronic azimuth mark is somewhat more com- 
plex in analysis than is the setting of the range mark. 
The true size of a radar reference is the same for one 
range as another. Therefore, it would be expected 
that at great range the angle subtended by the refer- 
ence would be small, and the angular accuracy with 
which the angle may be bisected is high compared 
with that of larger angles. On the other hand, if the 
radar beamwidth were assumed to subtend a constant 
angle, the width of a radar signal would be a function 
of the range multiplied by the angular beamwidth. 
This means that the farther the target, the broader 
its appearance on the oscilloscope and the lower the 
accuracy of bisecting. Actually, the width of the 
signal on the PPI represents target width plus the 
apparent radar beamwidth at the range in question. 
Since most antenna patterns do not achieve the ideal 
of constant echo signal regardless of range, the radar 
angular beamwidth appears to be less at great ranges 
than at short ranges. Accordingly, the linear width 
of a signal does not appear in general to increase 
greatly at great ranges. At close range, however, the 
angular width is greater. This assumes that the target 
echo itself remains constant in width. However, since 
the reflecting surfaces of various targets are different, 
the apparent target width may vary considerably 
with range. 

Since the results are very much dependent upon 
the nature of the target, it is necessary to exercise 
care in selecting radar landmarks to be used for 
GPI fixes. 52bc When point targets are used as refer- 
ences, the azimuth resolution of the system is the 
determining factor for setability. The results of ex- 
perimental work by the authors indicate that the 
setability is approximately ±0.6 degree on point 
targets when GPI is used with an AN/APS-15 radar. 
This would indicate that with an excellent target 
located at ten miles, the azimuth fix error would be 
±0.1 mile. Thus, it is not unreasonable to expect 
wind-determination errors of about 6 to 10 miles per 
hour from this source. 

In addition to the range and azimuth errors just 
discussed, errors which will either add to or subtract 
from these may be caused by the use of PPI tubes 




GPI OPERATIONAL TECHNIQUE 


101 


with long-persistent screens. For example, if the 
GPI aircraft is traveling at a ground rate of 2P0 mph, 
the edge of a signal will move a distance on the PPI 
corresponding to 0.16 mile during the period of one 
revolution of a radar scanner rotating at 20 rpm. 
If the persistence of the tube is great enough to 
preserve one picture until the next is painted, then 
the targets will have indistinct edges of approxi- 
mately 0.1 mile in the direction of aircraft travel. 
Since all fix comparisons are made at the time when 
the radar scanner is pointed at the target in question, 
this particular effect of persistence only causes errors 
through operator confusion. However, even these 
errors could be eliminated by the use of rapid scan 
radar systems employing low-persistence PPPs. 

One effect of the roll and pitch of an aircraft is a 
blurring of the picture caused by motion of the signal 
on the PPI. Unfortunately, the simple remedy of 
placing the cross hairs on the signal, at the time 
when the signal is being painted on the oscilloscope 
screen, is not effective in this case. This is because 
roll and pitch actually make the target appear at a 
wrong azimuth position and therefore give rise to 
fix error. The only effective solution to this problem 
(if GPI is to be used in aircraft where roll and pitch 
are prevalent) is to stabilize the radar antenna or 
the GPI cross hairs to compensate for motion of the 
aircraft. 


9.3.4 Summary of Requirements 

The GPI computers which have been designed for 
aircraft are capable of navigating and bombing with 
precision equal to and with facility exceeding that of 
other devices used in the past. It should be pointed 
out, however, that the reliability of the information 
supplied by a machine such as GPI can never exceed 
the reliability of the data used in calculating this 
information. With the advent of high-speed aircraft 
and the trend to offset bombing techniques it is en- 
tirely possible that the errors in airspeed meters, 
compasses and other devices which are now toler- 
able, may become intolerable. 

9.4 GPI OPERATIONAL TECHNIQUE 

9.4.1 Navigational Procedure 

The practice of GPI navigation is outlined in the 
following four steps: 

1. Determine radar altitude. 

2. Measure wind. 


3. Set present position (fix) dials to indicate posi- 
tion of aircraft. 

4. Proceed with check point navigation. 

1. The radar altitude is found by pushing in the 
altitude knob of the GPI and turning until the range 
mark is coincident with the innermost ground signal 
on the PPI scope. 

2. Wind is measured and entered into the com- 
puter by first setting the GPI cross hairs on any 
prominent radar signal on the PPI. (The signal 
should be one which will not change in appearance 
during the next 1 to 3 minutes.) A switch button is 
pressed, and after a short waiting period, during 
which the cross hairs may drift away from the radar 
signal, thej r are reset on it. Thereafter, so long as 
the wind does not change, the cross hairs should re- 
main in coincidence with any radar reference signal 
on which they are set. 

3. The fix dials indicate, in N-S and E-W com- 
ponents, the ground range and direction to any 
reference on which the cross hairs are set. In Figure 
7, the fix dials (outer dials) show that the cross hairs 
are set on a point which is 18 miles north and 11 



Figure 7. Procedure for check-point navigation. 


miles east of the aircraft. If the aircraft should pro- 
ceed to this point the zeros of the fix dials will then 
have moved under the cursor while the cross hairs on 
the PPI will have moved into the center of the 
oscilloscope. 


iESTIUCTED^ 


102 


THE GPI BOMBING AND NAVIGATION COMPUTER 


The zeros of the fix dials may thus be considered as 
representing the PPI cross hairs. Now suppose the 
cross hairs to be set on a reference whose mileage co- 
ordinates are known with respect to the home base. 
If the present position dials are then turned until 
these mileage coordinates lie opposite the fix dials’ 
zeros (cross hairs), the present position of the aircraft 
with respect to the home base will be indicated on 
the present position dials under the cursor (normally, 
the present position and fix dials turn together) . 

In the example of Figure 7, the cross hairs have 
been set on the PPI to a point which is known to be 
118 miles north and 61 miles east of the home base. 
The fix dials indicate that this point is 18 miles north 
and 11 miles east of the aircraft. This means that 
the aircraft is 118 — 18 = 100 miles north and 
61 — 11 = 50 miles east of the home base. In Figure 
7 the present position dials have been set so that the 
coordinates of the known point (118 N and 61 E) lie 
opposite the zeros of the fix dials. The present posi- 
tion of the aircraft is now indicated under the cursor 
on present position dials. This constitutes step three 
of the navigational procedure. 

4. The check-point system of navigation permits 
the use of the pilot’s direction indicator [PD I] meter 
for indicating the heading the aircraft must follow in 
order to pursue a given ground track. The mileage 
coordinates with respect to the home base of any 
convenient point which lies on the desired ground 
track are selected by reference to an aeronautical 
chart or other source. This point is one over which 
the aircraft should pass in order to be on course (see 
Figure 7), and it need not reflect a radar signal. The 
fix dials are then turned until their zeros come oppo- 
site the mileage coordinates of the point in question. 
This operation places the cross hairs over this point 
on the PPI scope even though there may be no radar 
signal. If the function switch is now turned to the 
course position (C), the PDI meter will indicate a 
heading for the aircraft to fly which is upwind from 
the desired ground track by an amount equal to the 
drift angle. In addition, the time-to-go [TTG] meter 
will indicate the time in minutes before arrival at the 
selected point. This procedure of setting up check 
points for the GPI may be repeated as many times 
as desired until the aircraft arrives at its destination. 
If the aircraft has wandered off course during a time 
when the GPI may have been left unattended, the 
setting up of a single check point will bring it back to 
the prescribed ground track, and a second check 
point will cause the aircraft to be turned on course. 


If the check-point system is used, and if a new wind 
determjnation is made every time there is reason to 
believe it has changed, a predetermined course can be 
flown very accurately. 

9.4.2 Identification of Radar Signals 

Radar signals appearing on a PPI tube can be 
identified by use of the GPI. One procedure for this 
would be to place the cross hairs on the unknown 
signal and then to read the mileage coordinates of 
that signal from the present position dials opposite 
the zeros of the fix dials. By reference to a navigation 
chart showing these coordinates, the geographic loca- 
tion of the unknown signal can be found. Conversely, 
if it were desired to select a particular signal appear- 
ing among a number of other signals on the PPI, it is 
only necessary to set the fix dial zeros opposite the 
mileage coordinates of the desired signal on the 
present position dials. Then the cross hairs will 
appear over the desired signal on the PPI. 

9.4.3 Classification of Reference 

Points for GPI 

The GPI technique divides reference points into 
three categories: 

1. Principal references are those which by nature 
of their shape or position relative to other signals may 
be easily and positively identified by the use of radar 
with ordinary charts. The use of GPI is not essential 
for these references. Principal references are used to 
set the present position dials at the beginning of the 
navigational procedure. Since GPI is subject to a 
cumulative position error, the present position dials 
may have to be reset occasionally. This should be done 
only when the reference can be positively identified. 
The importance of using only principal references to 
set the present position dials is stressed. The philos- 
ophy behind this is that in the absence of positive 
identification, the best information available is that 
stored in the GPI. 

2. Intermediate references are those which appear 
as good radar signals but are not easily identified 
without a complete knowledge of present position. 
GPI use is essential for these references. 

3. Local references are those which appear as 
small, weak radar signals. There are usually many 
of these unidentified signals appearing on the PPI. 
One of these can be picked up and identified by 


RESTRICTED,-: 


GPI OPERATIONAL TECHNIQUE 


103 


manipulating the fix dials to known coordinates of 
the reference and allowing the cross hairs to identify 
it on the radar scope. The destination of the aircraft 
may well possess such characteristics. If the cross 
hairs be set to the destination signal on the radar, the 
GPI will compute a very accurate estimate of ar- 
rival time. 

In general it should be stated that only some 
known discrete point of a reference should be used for 
fix or wind determination. The remaining larger por- 
tion of the reference signal only serves as an identi- 
fication aid. 

9.4.4 Bombing Procedure 

Having navigated to and identified the target, 
there are two methods of bombing it. If it is a target 
which is small in size and gives a discrete radar 
signal, it is no doubt best to bomb by direct bombing- 
techniques, since greater computation accuracy can 
be achieved in this way. However, if the target is 
complex or gives either a very poor radar signal or 
none at all, then it is better to use the offset bombing 
technique. 

The process used for direct bombing consists of 
placing the cross hairs on the target desired, making 
sure that they are synchronized with the target, then 
permit ting the pilot to fly by means of the PD I until 
bomb release point is reached. Before the bomb run, 
the radar-bombardier would have set the values for 
time of fall and trail into the computer by referring 
to ballistic tables. For the particular GPI described 
herein, there exists the disadvantage that the pilot 
cannot use PDI and TTG information while the 
bombardier is making cross hair adjustments. How- 
ever, this is a fault of one particular GPI which can 
be removed in future designs. This limitation is not 
serious when the GPI is used for navigation, but be- 
comes a definite handicap in bombing, where time is 
important. 

The process used for offset bombing is the same as 
for direct bombing so far as the bombardier is con- 
cerned. However, the point used for aiming in this 
case is actually some radar reference other than the 
target. Offset bombing is used when there are no well- 
defined characteristics on the target which can be 
used as aiming points, since it permits the choice of a 
radar reference signal which is easily recognized and 


well defined. Using this reference signal and the offset 
technique, a more accurate determination of the 
position of the target can be made than is otherwise 
possible. This facilitates the bombing of complex or 
invisible targets and helps overcome the difficulty of 
finding suitable radar echoes which was referred to 
in Section 9.4.3. 

9.4.5 Future GPI Techniques 

The description and procedures of the preceding 
sections are concerned with a specific version of GPI. 
This GPI uses a rectangular coordinate system of 
computing and presenting data and has the char- 
acteristic that it works with fixes consisting of range 
and azimuth to a simple point. This is in contrast to 
other possible GPUs which might, for example, work 
on the basis of range measurements only to each of 
two beacons or other known points. Furthermore, 
this GPI requires the operator to judge when changes 
in wind make necessary a new computation of wind. 
There exist in the design stage several new types of 
GPI which are not so restricted. 

Designs have also been made that permit present 
position to be given as degrees of latitude and longi- 
tude. Identification can also be made in latitude and 
longitude while the fix distance is given in miles from 
the aircraft to the reference. The major change that 
is made for a latitude-longitude presentation is multi- 
plication of the E-W rates by the secant of the 
latitude in degrees. 

It is desirable and rather simple to provide a sys- 
tem of counters and differentials in place of dials. 
With a system of this type there would be three sets 
of counters: one giving fix distance; a second giving 
present position of the aircraft; and the third giving 
the position of the cross hairs relative to the home 
base. The third set could be labeled identification. 
The counters would then be used in the same way as 
the dials, but their presentation would be clearer. 

Other refinements of GPI methods will almost surely 
include continuously automatic altitude measure- 
ment. Operatorless GPI’s are now (1945) being de- 
signed which solve continuously all problems of navi- 
gation, including the determination of drift angle. 
This type of GPI does not require a search radar but 
can use sonic or radar doppler principles, or beacons, 
to obtain continuous ground speed and drift data. 


RESTRICTED 



Chapter 10 


BEACON 

10.1 BEACON BOMBING SYSTEMS 

10.1.1 Introduction 

The most precise bombing so far done with radar 
has involved the use of beacons (racons). There are 
two primary advantages in bombing on a beacon re- 
turn pulse rather than on a radar echo. The first 
advantage is that the radar return from a target is 
often ill-defined because the echo signals may fluctu- 
ate considerably as the aircraft moves, or no sharp 
boundary of the radar reflection may exist. A beacon 
return comes always from the same place and has a 
leading edge that is clearly defined. The second ad- 
vantage is that the use of a beacon eliminates the 
vexing problem of target identification. There is no 
possibility of confusion with surrounding ground 
echoes (clutter) and most bombing beacons are coded 
so that misidentification of the beacon is very un- 
likely. 

An additional advantage of most beacon bombing 
schemes is that only range measurements are in- 
volved. Radar azimuth measurements, which are in 
general less precise than range measurements, need 
not be used. The ranges to two beacons are measured 
and this furnishes the data needed to fix the position 
of the aircraft in space. 

Several serious disadvantages are, however, in- 
herent in beacon blind bombing. The outstanding 
drawback is that the aircraft must bomb within the 
radar horizon distance of the cooperating microwave 
beacon or radar. Assuming that the ground equip- 
ment is located just inside friendly territory, an 
aircraft at 30,000 ft cannot penetrate further than 
250 miles into enemy territory; an aircraft at 12,000 
ft must bomb within 160 miles of the front line. 

A second complication in beacon bombing is that 
the distance from the ground equipment to the target 
(more correctly, to the bomb release point) must be 
very accurately known. This calls for accurate map- 
ping of the combat area, and in many parts of the 
world sufficiently good maps do not exist. The alter- 
native is the use of the bombing system as a mapping 
device, flying first a reconnaissance mission and then 
the combat mission. In either case, the geographical 
location of the target must be known precisely, which 
calls for careful intelligence work, and the ground 
equipment must be accurately sited. A large amount 
of calculation is necessary after the geographical 
coordinates have been determined, in order to ascer- 


BOMBING 

tain the desired ranges. The calculations take time, 
which is usually at a premium before a bombing 
mission. (On the other hand, the absence of target 
identification problems helps to reduce the necessary 
briefing time.) 

With most beacon bombing schemes used so far, 
technical limitations restrict the possible approaches 
to the target to only a few directions. This is a dis- 
advantage, since such operational factors as enemy 
flak and weather may indicate a preferred direction 
of approach which is not technically possible. 

10.1.2 Types of Beacon Bombing 
Systems 

Three types of beacon bombing systems will be 
considered as well as one type of bombing system 
which does not involve beacons but is in several re- 
spects similar. 

H Bombing Systems 

In an H system (see Figure 1) the aircraft carries 
either a radar or some other pulse ranging device. 
Two beacons are located at known points on the 
ground. A transmitted pulse from the aircraft trips 
both beacons, and the beacon replies are received in 



Figure 1 . H bombing system. 

the aircraft. The range to each of the beacons is de- 
termined and the aircraft position is thus established. 
The airborne equipment contains devices to indicate 
to the pilot whether he is to the left or right of the 
proper course, and to indicate to the bombardier or 
bomb release mechanism when the aircraft reaches 
the bomb release point. 

Specific advantages of H systems as contrasted 
with other beacon bombing schemes are the sim- 
plicity of ground equipment and liaison requirements 


RESTRICTED 


104 


BEACON BOMBING SYSTEMS 


105 


and the large traffic handling capacity. The simple re- 
quirements of the beacons are that they reply to 
every interrogating pulse and that their positions be 
accurately known. The operation of the ground 
equipment, once sited, is fully automatic, and no 
preparation other than normal maintenance is neces- 
sary for operational use. Since the beacons respond 
to every aircraft equipped with proper interrogators, 
the only limitation on the total number of aircraft 
that can be simultaneously controlled is overloading 
of the beacon. In general, tens to hundreds of aircraft 
can obtain satisfactory replies simultaneously. 

The disadvantages peculiar to H systems stem 
from the need of precise range measurements in the 
aircraft. Either elaborate apparatus, or a highly 
skilled operator, or both, must be carried for good 
results. Since, in general, airborne equipment must 
be small, light, and rugged, it is not possible to have 
as accurate or as fully automatic ranging apparatus 
in the air as on the ground. Moreover the operator, 
however skilled, has to contend with the distractions 
of flight noise, oxygen equipment, crew duties, and 
enemy action. 

Oboe Bombing Systems 

In an Oboe beacon bombing system (see Figure 2) 
a beacon is carried in the aircraft and two ground 
radar stations make range measurements on the 


aircraft’s position in space. Steering and bomb release 
information is given to the aircraft by coding the 
radar interrogating pulses and incorporating suitable 
decoding equipment with the airborne beacon. 

Oboe systems have the advantage that the ranging 
is done by skilled operators on the ground, free from 
combat distraction. Moreover, the ground radar set 
may be as large, as intricate, and as complex as de- 
sired. 

The disadvantages are that a given pair of ground 
stations can handle only one aircraft at a time, so the 
only answer to the requirement for simultaneous 
attacks by many aircraft is the cumbersome one of 
building many pairs of ground stations. Further- 
more, before each mission both ground stations must 
receive, rapidly and accurately, through secure 
channels, all information on the time and plan of 
attack and the precise ranges to the target. In prac- 
tice, a complex and difficult liaison problem arises. 

Beacon Offset Bombing 

In this type of bombing (see Figure 3), the aircraft 
carries a standard bombing radar. A ground beacon, 

AIRCRAFT WITH 

BOMBING RADAR 


AIRCRAFT rV»~--si=v. 
CARRYING 



Figure 2. Oboe bombing system. 

beacon. Here again the positions of the ground sta- 
tions are accurately known, so the ranges from the 
two stations plus the aircraft’s altitude define the 


'TOO YDS 

TARGET 

N 

Figure 3. Beacon offset bombing. 

usually of the ultra-portable type, is placed at a 
known position in relation to the target. In the 
simplest form, the aircraft flies over the beacon on 
the beacon-target heading and releases bombs at 
the correct range from the beacon. In more elaborate 
forms, an offset bombing computer or schemes in- 
volving more than one beacon may be used. 

Advantages of this system are that no airborne 
equipment is required other than a standard bomb- 
ing radar and perhaps a stopwatch; also, the system 
can be set up rapidly under battle conditions. It is 
conceivable, for example, that a ground cooperation 
officer could call an aircraft flying overhead and ask 


PORTABL 

BEACON 


d — 



106 


BEACON BOMBING 


that three bombs be dropped on a point 1,700 yd at 
278 degrees from Marker 6. 

Since it involves azimuth measurements, this bea- 
con offset system has the disadvantage relative to 
other beacon bombing systems that angles must be 
measured. Because of this, accurate bombing can be 
done only within a few miles of the beacon. The 
flying problems involved call for considerable skill 
and judgment and proper coordination of the activi- 
ties of the beacon crew presents an organizational 
problem. 

Hyperbolic Navigation Systems Used for Blind 
Bombing 

Radio navigation systems such as British Gee and 
American SS Loran (see Figure 4) have been used 
for blind bombing. These are not radar and are not 
beacon ranging systems, but since they are ground- 



based pulse systems it seems appropriate to discuss 
them here. The principles underlying hyperbolic 
navigation are discussed elsewhere. 55 Instead of 
ranges, time differences are measured; one time-dif- 
ference reading from one synchronized pair of ground 
stations establishing a line of position. Two time- 
difference readings, from two different pairs of ground 
stations, give a fix by the crossing of their two posi- 
tion lines. The accuracy of the fix depends on the 
angles subtended at the aircraft by each pair of 
ground stations, and on the angle at which the posi- 
tion lines cross. 


It is of interest to compare the theoretical accuracy 
limitations of ranging systems such as H or Oboe 
with those of the hyperbolic Gee and SS Loran sys- 
tems. If a Gee system involving three ground stations 
is compared with a ranging system whose two bea- 
cons are located on the sites of the outer Gee stations, 
the following results will be found: first, the radial 
accuracy of the ranging system is always consider- 
ably superior since range is measured directly; sec- 
ond, the tangential accuracy is usually somewhat 
better for the ranging system (it is poorer near the 
line joining the two stations); third, the positional 
accuracy of the ranging systems falls off less rapidly 
with increasing range, being better in accuracy than 
the Gee system at all except the very shortest ranges. 

Ordinary Loran is subject to the same limitations 
as Gee and has never been used for blind bombing 
although SS Loran has been used for bombing. With 
sky-wave synchronized [SS] Loran, the station sepa- 
rations are of the order of ten to fifteen times those 
used in Gee, and the ordinary geometrical limitations 
are almost entirely avoided. The four stations are 
located on various sides 'of the target area instead of 
at one side, and maximum accuracy is obtained near 
the center of this area instead of near the stations. 
The stations are synchronized by sky waves which 
are present only at night. The errors introduced by 
the variable time of propagation of these reflected 
sky waves result in average position errors of about 
a mile, but the service area is approximately 1,000,000 
square miles. It should be noted that the same type 
of airborne equipment is used for Loran and SS 
Loran. 

Gee and the ranging systems are limited in range 
to horizon distances, or a few tens of miles beyond, 
by reason of the high radio frequencies used. Even at 
high altitudes the coverage area rarely exceeds 75,000 
square miles. 

A further advantage characterizing hyperbolic sys- 
tems is that no airborne transmitter is used, which 
simplifies equipment and makes the traffic capacity 
unlimited. The principal advantage of ranging sys- 
tems lies in their high degree of precision within 
their useful range. 

10.2 H BOMBING 

10.2.1 General Characteristics of 
H Systems 

A representative block diagram of an H system is 
shown in Figure 5. The airborne range unit generates 


H BOMBING 


107 


a pulse used as the zero of the time base, which 
triggers the transmitter. The transmitter pulse is 
radiated and may or may not be coded to distinguish 


RECEIVER TRANSMITTING 
ANTENNA ANTENNA 



BEACON B 

Figure 5. Typical H system block diagram. 


it from other pulses of the same frequency. This pulse 
is received by both beacons. The beacon reply, coded 
for identification purposes, is received in the aircraft 
and timed by the range unit which passes both 
beacon ranges to a computer. The computer output 
gives the air crew or automatic equipment the in- 
formation necessary to fly the correct course and to 
release bombs. 


range, plus the delay in the beacon multiplied by one- 
half the velocity of light. The beacon delay can be 
measured and taken into consideration but any vari- 
ation in the delay appears as a range error. Such vari- 
ations in delay are caused by changes in received 
signal strength, duty cycle of the beacon, temperature 
and line voltage fluctuations; and the aging of circuit 
elements; and they must be minimized. 538 Some bea- 
cons have provision for monitoring the delay and re- 
adjusting it from time to time to a predetermined 
value. To avoid continual tuning of the airborne re- 
ceiver, the beacon frequency should remain very close 
to its assigned value. 

The beacon receiver need not be so sensitive as the 
best radar receivers, since the signal it receives de- 
creases according to the inverse square instead of in- 
verse fourth power law (see Chapter 7). It should 
accommodate a somewhat broader frequency band, 
however, in order to receive signals from a number of 
radar transmitters which may spread in frequency. 

Airborne Ranging Unit 

For comparable absolute accuracy, the require- 
ments on an H system range unit are more stringent 
than for any other radar equipment. Since ranges 
must be measured to within a few yards at distances 
of hundreds of miles, the range-unit crystal fre- 
quency should be accurate to within at least one part 
in one hundred thousand. Tolerances on circuit ele- 
ments used to convert crystal oscillations to con- 
tinuously variable time measurements must be care- 
fully chosen to remain within the allowable limits of 
error. Since it is airborne, the unit must also be light, 
compact, and stable under varying conditions of 
temperature and pressure. The range unit must be 
capable of making measurements on two beacons, 
either by use of simultaneous channels, or by rapidly 
switching from one beacon to the other during the 
period of rotation of the scanner. 

Airborne Transmitter 

The transmitter requirements are those for a stand- 
ard radar transmitter which should produce a pulse 
with a rapidly-rising leading edge. 


Beacons 


A prime requirement for the beacons is that the 
delay (the time between the reception of the leading 
edge of the interrogating pulse and the transmitting 
of the leading edge of the reply) be constant. The 
range measured at the aircraft is the aircraft-beacon 


Airborne Receiver 

As with the beacon receiver, the ultimate in sensi- 
tivity is not required, but it is necessary that the 
receiver preserve the shape of the radio-frequency 
pulse, so that time measurements made on a char- 



RESTRICTEI) 



108 


BEACON BOMBING 


acteristic part of the pulse will be accurate. If both 
beacons reply on the same frequency, the bandwidth 
of the receiver should be sufficient to cover any ex- 
pected frequency spread between the two, caused by 
the beacon transmitters drifting off tune. If the bea- 
cons reply on differing frequencies, two receivers are 
required, at least through the last stage of radio- 
frequency amplification. 

Computer 

The function of the computer is to transform the 
range data on the two beacons into information in a 
form useful for flying the aircraft on the bombing run 
and releasing the bombs. Many degrees of computer 
complexity exist, extending from a simple presentation 
that indicates to an operator when a precomputed 
bomb release point has been reached, to computers 
capable of solving the bombing triangle while flying 
over the target, using data corrected to that mo- 
ment. 

Computers have been made which take into ac- 
count, in the solution of the bombing problem, some 
or all of the following. 

1. Ground speed measured on the bombing run 
(by determining rate of change of beacon range) . 

2. Ballistics of particular type of bomb. 

3. Correction for wind found on bombing run. 

4. The preferred direction of approach. (Some 
computers allow several directions of approach; 
others allow an arbitrary direction of approach.) 

5. Evasive action after the aircraft is committed 
to the bombing run. 

6. Correction for curvature of a nonlinear course 
(since the bombs fall on a tangent to the aircraft 
course). 

7. Changes in altitude and airspeed. 

Indicators 

Indicators are required to show the distance of the 
airplane to the right or left of the correct course, and, 
in more refined systems, to show whether the aircraft 
has the correct heading. The instant of bomb release 
must be indicated to the bombardier. Indicators can 
be dispensed with in wholly automatic systems where 
the output of the computer is connected both to the 
autopilot for actually flying the aircraft, and to the 
bomb release mechanism to actuate it at the correct 
moment. The application of this latter type of sys- 
tem to guided missiles and pilotless aircraft is ob- 
vious. 


10.2.2 Gee-H 

The most extensively used H system of World War 
II was Gee-H. As the name suggests, the system 


TRANSMITTING 

ANTENNA 



Figure 6. Block diagram Gee-H airborne system. 


operates in the same frequency range and with much 
of the same equipment as the Gee navigational sys- 
tem. In fact, the equipment (A.R.I. 5525) could be 
used either for Gee navigation or H bombing as de- 
sired. 

A block diagram of the airborne system is given in 
Figure 6. A crystal-controlled oscillator in the indi- 
cator is connected to dividing blocking oscillators and 
multivibrators to generate a trigger at the normal 
Gee pulse recurrence frequency of 500 c. To reduce the 
duty cycle of the beacons, a “ blackout unit” is used 
which admits two successive triggers to the modu- 
lator and indicator sweeps, and then blocks out the 
next eight triggers. The modulator energizes the 
transmitter which transmits a 2-jusec pulse at a fre- 
quency somewhere between 22.5 to 30 me. The bea- 
cons reply in the frequency range 50 to 60 me. 

The received pulses in the aircraft are displayed on 
either a 350-mile scale, or an expanded time base of 
approximately 15 miles, as desired. The two locally 
transmitted interrogating pulses also appear on the 
oscilloscope to provide the zero reference for the de- 
lay measurements. A double-trace display makes it 
possible to make simultaneous measurements on the 
two beacon return signals. 

Another feature of this system is that a jitter cir- 
cuit in the blackout unit desynchronizes the dividing 
blocking oscillator during blackout periods. If two 
interrogating aircraft happened to have pulses ar- 
riving at a beacon very close together (within 50 /isec 
or so), the beacon would reply to the first and, not 
having had time to recover, ignore the second. Since 


RESTRICTED 



II BOMBING 


109 


both aircraft operate from crystals agreeing in fre- 
quency very closely, if one such coincidence occurs, 
the next pair of pulses would also be close enough to 
interfere, and so on. To avoid this, the jitter circuit 
can vary the recurrence frequency slightly during the 
blackout period. 

The airborne crystal frequency, on which the ac- 
curacy of range measurement depends, can be 
matched against the frequency of a carefully main- 
tained standard crystal at the ground Gee station. 
Gee navigation cover is always provided in areas of 
H operation, and since Gee navigation is usually used 
in the first phases of an attack, the airborne crystal is 
automatically set to the accuracy of the Gee repeti- 
tion frequency. The same accuracy then applies to 
the H ranging if the crystal trimming adjustment is 
not disturbed when changing from Gee to H opera- 
tion. 

In the initial design of Gee-H there was no air- 
borne computer. The Gee-H operator in the aircraft 
observed the beacon signals on the indicator. He 
gave verbal instructions to the pilot to maintain the 
appropriate range from one beacon and to the bom- 
bardier to release bombs at the correct range from the 
other beacon. The bomb release point was precom- 
puted from ballistic data, meteorological predictions, 
and briefed flight instructions. Later, the Royal Air 
Force developed a simple reversible clock computer 
to take account of ground speed, and the 8th Air 
Force developed a check point (see Section 8.3.1) 
method of synchronizing the Norden bombsight to 
calculate ground speed. 

Two types of ground beacons were developed. The 
“ heavy” beacon is a permanent installation, requir- 
ing a total maintenance and operating group of per- 
haps 25 people. The transmitter output was rated at 
peak 50 kw'. A “lightweight transportable” beacon, 
rated at 20 peak kw, was designed in units which 
could be hand-carried, the entire equipment being a 
small truck load. A total personnel complement of 
fifteen was required for this type of beacon, which 
could also be used as a Gee ground station. Both the 
heavy and the light transportable beacons were con- 
tinuously monitored. 

One advantage of Gee-H is the extended range 
made possible by the relatively low frequency. Ex- 
treme ranges of 300 miles on aircraft, flying at 17,000 
ft, have been reported. The alternative use of the 
equipment as a standard navigational aid w r as very 
useful. The ability to check and adjust the crystal 
frequency at wfill, as well as the relative simplicity 


of the airborne equipment, are other advantages of 
Gee-H. Moreover a pair of the heavy ground bea- 
cons could handle 80 aircraft simultaneously, which 
is more traffic than the majority of H systems can 
accommodate. 

One major disadvantage of Gee-H results from the 
use of low 7 -frequency radiation. The transmitted pulse 
has a time of rise of the order of a microsecond, and 
because the receiver video pulse must rise to some 
threshold value before it trips the beacon or appears 
on the airborne indicator, the measured range will 
vary with signal strength. A crude compensation for 
this variation can be made by increasing the video 
gain of the receiver for weak signals, but the beacon 
can make this adjustment for only one set of signals 
at a time, and if more than one aircraft is interrogat- 
ing the beacon, some compromise must be reached. 
Constant monitoring of the received signal strength is 
needed, and, even so, ranges read at a particular air- 
craft may be hundreds of feet in error. A further 
difficulty is frequency drift in airborne transmitters 
so that the beacon receiver cannot be tuned correctly 
to all aircraft simultaneously. This fact causes further 
pulse distortion, and consequently delay variation. 

Another disadvantage is the need for the operator 
to give directions to the pilot over the interphone to 
keep him on course, which is not easy. A further dis- 
advantage is the crudeness of the airborne computer 
which at best corrects for ground speed only. Finally, 
the ground equipment requires a cumbersome amount 
of attention. 

Gee-H was introduced in operations by the RAF 
Bomber Command in the fall of 1943, and w T as used 
extensively by that organization and RAF Second 
Tactical Air Force for the last eight months of the 
European War. In the spring of 1944 the 8th Air 
Force started operations with sufficient Gee-H Path- 
finders to lead tw r o of its combat wings. Bombing ac- 
curacy in training was of the order of }/i mile, and in 
combat perhaps 0.8 mile circular probable error. 

to.2.3 Micro-H 

Micro-H is an H bombing scheme designed for use 
with aircraft which carry bombing radars. The prime 
advantage of the system is that it enables aircraft 
equipped with H2X (AN/APS-15, AN/APS-15A, 
and AN/APQ-13) radar systems to use the more pre- 
cise II bombing technique within its horizon limita- 
tions. In the Mark I version of Micro-H no additional 
airborne equipment is required, wLile in the later 


jtfEST 


TRICTED 


m 




110 


BEACON BOMBING 


versions only relatively minor attachments are 
needed. 

Micro-H Mark I 

The H2X radar systems were equipped for 
navigational purposes to receive signals from the 
AN/CPN-6 radar beacons. The H2X range unit, 
since it is used to measure range to a radar target, 
can make fairly precise range measurements on a 
single beacon. It would be possible, therefore, to do 
a type of H bombing by locating the beacons so that 
the target is equidistant from the two beacons. By 
flying a course such that the beacon-aircraft ranges 
remain equal, the bombing aircraft would pass over 
the target, and bombs could be dropped when both 
beacons were at the correct releasing range. 

Such a scheme would involve the use of no addi- 
tional airborne equipment or of ground equipment 
other than standard navigational beacons. The siting 
requirement would be impossibly cumbersome in 
practice. However, the same result can be obtained 
by inserting an artificial delay time in one beacon. 

Suppose, for example, that beacons are sited in a 
combat area and it is desired to attack a target which 
is found to be 185.63 nautical miles from Beacon A 


/ 

/ 

/ 



and 147.28 miles from Beacon B (see Figure 7). Let 
the differences in these ranges, 38.35 miles (or 411 


Msec) be inserted as an added delay in Beacon B. 
Beacon B then appears to the aircraft radar to be 
38.35 miles further than its actual range. If the air- 
craft flies a course which keeps both beacons at ap- 
parently equal ranges, its course will be a branch of 
a hyperbola passing over the target. 

The several components of the Micro-H Mark I 
system are : 

Beacons. Standard AN/CPN-6 beacons (or the pi- 
lot production prototype, the CXEH beacons) are 
modified by adding a supersonic delay line between 
the receiver and the coder. By varying the length of 
the water path through which the supersonic pulse 
passes, it is possible to add a delay of any value be- 
tween zero and 90 miles to the beacon response, when 
using a delay tank that is 80 in. long. 

The beacon receiver discriminates between pulses 
of different duration so that it responds only to 
pulses of width greater than 1.9 jusec, and hence does 
not reply to the usual one-half or one microsecond 
radar search pulses. The beacon reply is coded by 
using from two to six pulses, with long or short 
spacings between pulses. The number of pulses and 
the choice of spacings identifies the beacon. Range 
measurements are made on the leading edge of the 
first reply pulse. The beacon receiver has a pass band 
1 10 megacycles wide, to accept all signals within the 
frequency band assigned to H2X. The beacon trans- 
mitter output is rated at 40 peak kw. Test equip- 
ment is built into the beacon station for monitoring 
frequencies, signal strength, and delay. Mobile bea- 
con stations have been made by mounting an oper- 
ating beacon and a spare beacon in a van with main- 
tenance facilities. 

Airborne Equipment. The airborne equipment is an 
unmodified AN/APS-15 or AN/ APS- 15 A radar sys- 
tem, which will not be described here except to point 
out some of its limitations as Micro-H bombing 
equipment. Since the airborne receiver is a high gain, 
narrow band radar receiver, it requires frequent ad- 
justment for optimum beacon reception (unless the 
system has beacon AFC such as more recently de- 
signed systems have). Some of the r-f components, 
since they are sharply tuned for radar echo reception 
on a fixed frequency rather than for beacon fre- 
quency, are apt to be 15 to 20 db below maximum 
sensitivity at beacon frequency. 

The indicator employed is the standard H2X plan 
position indicator [PPI], On the bombing run it is 
usually used with a 15-mile sweep, the target being at 
the extreme range of the display. A range circle of 


RE! 


RESTRICTED 


H BOMBING 


111 


adjustable, known range is displayed on the PPI. 
The radar operator varies the range of this reference 
circle until it becomes tangent to the signal from one 
beacon. He then gives the pilot instructions over the 
interphone to fly the airplane so that the other 
beacon return also becomes tangent to the reference 
circle. He also notifies the bombardier when ranges 
corresponding to precomputed check points have 
been reached, and the bombardier uses this informa- 
tion to synchronize the Norden bombsight (in a 
manner analogous to that described in Section 8.3.1). 
Since the PPI range scale is 6 miles per inch, it is very 
difficult, although possible, to judge tangency to the 
reference signal (and hence to determine range) closer 
than 100 yd. Furthermore, experience and skill on the 
part of the radar operator are required to interpret an 
off-course indication and to give correct steering in- 
structions for the pilot. 

The radar range unit (see Section 7.3) uses a 
crystal-controlled oscillator with subsequent divid- 
ing stages so that any integral multiple of 10 nautical 
miles can be used for ranging. To this is added an 
additional delay, variable from zero to 10 miles, to 
provide the zero of the time base. A third inde- 
pendent delay, variable between zero and 15 miles, 
can be added to obtain the reference range mark on 
the PPI. With very careful calibration and main- 
tenance, it is possible to determine ranges to within 
50 ft in the placing of the leading edge of the refer- 
ence mark. 

Discussion of Micro-H Mark I. The main disadvan- 
tage of Micro-H Mark I is the substantial demand 
made on the skill and attention of the radar operator 
to keep the equipment in adjustment, to interpret 
the scope presentation correctly, and to direct the 
pilot and bombardier. Neither in combat nor in prac- 
tice was the limiting accuracy of the system ap- 
proached, which indicates that the instrumentation 
was insufficient to make full use of the available data. 

A second disadvantage is that information on range 
to the beacons is received only once every 3 seconds, 
which is the period of the airborne scanner rotation. 
For example, an aircraft moving at 300 miles per 
hour moves 34 mile between scans. On the other hand, 
the time required to make changes in aircraft course 
is about 3 sec, so more frequent information may not 
be useful. The 3-sec figure can be halved by the use 
of sector scanning. 

Micro-H Mark I is not exactly a true H system be- 
cause the beacon delays need to be adjusted for each 
particular target. This means it is necessary to have 


communication with the beacons before a mission, 
which is a serious limitation when the beacons are in 
forward areas. Furthermore, while many aircraft can 
use the beacons simultaneously, only one target can 
be attacked at a time. 

The great advantage of Micro-H Mark I is that no 
additional airborne equipment is required in aircraft 
carrying the H2X radar and the Norden bombsight. 
With this combination, the aircraft has its choice of 
visual bombing, pure radar bombing, or H bombing, 
as conditions dictate. The combination gives a bomb- 
ing force remarkable flexibility. 

Micro-H Mark I was introduced into operations 
by one division of the 8th Air Force in November, 
1944. It was used extensively by this division until 
the close of the European War. The average circular 
error in combat use was approximately 0.8 mile. 

Micro-H Mark II 

The Mark II version of Micro-H is designed to re- 
move the limitation to a single target which char- 
acterized Mark I. This is done by adding to the stand- 
ard LI2X radar an airborne attachment (AN/APA- 
40A). (See Figure 8.) Since the angle subtended by 

amtcmma /MICRO-H ANTENNA ATTACHMENT 


RECEIVER h- 


1 1 1 1 1 

t 

i r — i i 

, , .. 

J ! 

L-jTRANSMlTTERi--—*--»RANGE unit f- 

i i i 

"TT J 


MICRO-H 


SWITCHING 

BOX 


--icomputer! 

I I 

I J 


Figure 8. Block diagram Micro-H Mark II. Standard 
H2X equipment shown in dotted lines. Micro-H at- 
tachments shown in solid lines. 

the beacons at the aircraft is always larger than the 
angular width of the radar beam, the airborne radar 
interrogates first one and then the other beacon. It 
is therefore possible to switch between two sets of 
circuit constants in the range circuit so that the 
ranging is correct for one beacon when the antenna 
is pointed at that beacon and for the second beacon 
when the antenna is pointed at it. This is the prin- 
ciple of the AN/APA-40A attachment. 

The range unit delays the beginning of the indi- 
cator sweep independently of the range reference 
mark (see Figure 9) . This makes possible the addition 
of an apparent delay in one beacon. For example, in 
the case treated under Mark I, let the indicator sweep 


RESTRICTED 


112 


BEACON BOMBING 


start at 135 miles when the scanner is pointed at 
Beacon B. When Beacon A is interrogated, let the 
indicator sweep start at 135 + 38.35 = 173.35 miles. 
If the aircraft is now flown so that both beacons 
appear at apparently equal range on the indicator 


TRANSMITTER PULSE 


START OF SWEEP START OF SWEEP 
BEACON B BEACON A 


(ZERO TIME) 

A 

“5 


SWEEP 

■♦LENGTH-* 


RANGE UNIT 


A A 

135 


'r — ---i 

173,35 TIME BASE 



Figure 9. Timing diagram Micro-H Mark IT. 


(that is, so that the actual range minus the delay in 
the start of the sweep is the same for both beacons) 
it will follow the branch of the same hyperbola 
previously described in Figure 7. 

A block diagram of the system is shown in Figure 
8, while Figure 9 is a timing diagram showing the 
sequence of events in ranging on either beacon. From 
Figure 9, it is evident that, when flying a hyperbolic 
course, the display sweep is started at different times 
for the two beacons but the reference range mark 
occurs at the same time interval following the be- 
ginning of the sweep in each case. Figure 10 is a PPI 
photograph taken when an aircraft equipped with 
Micro-H Mark II is nearly on course and is ap- 
proaching a check point. 

It is also possible, with Micro-H Mark II, to fly a 
circular course about either beacon. In this case, a 
fixed reference mark is placed in the sector of the 
display containing one beacon, and a calibrated mov- 
able mark in the sector containing the other beacon. 
The course is flown so as to keep one beacon at con- 
stant range while precomputed check points de- 
termined on the other beacon are used to synchronize 
the Norden bombsight. 

An attachment on the scanner switches the circuit 
elements when passing from one sector to the other. 
When once adjusted this automatically performs the 
necessary switching at the correct azimuths in each 
scan. 

Mark II Micro-H has the advantages over Mark I 
that any number of targets may be simultaneously 
attacked, no communication with the beacon is re- 


quired in setting up a mission, and circular as well 
as hyperbolic courses are conveniently possible. It 
retains the disadvantage of Mark I in demanding 
very high operator skill. 

Micro-H Mark II was being introduced into oper- 
ations by the 8th Air Force at the close of the 
European phase of World War II. In its few practice 
and combat trials, it showed substantially the same 
accuracy as Micro-H Mark I. 

Micro-H Mark III 

Micro-H Mark III was designed for use with cir- 
cular courses only and is an attempt to ease the task 
of the radar operator in two ways: first, by switching 
the sweep expansion as well as the start of the sweep, 
the beacon at constant range can be viewed on a 
4-mile scale, while the 15-mile expansion required for 
the determination of check points is retained in the 
sector containing the other beacon; second, an indi- 
cation of angular deviation from course is provided, 
in addition to the information of displacement from 
course. Thus, the airborne operator can tell, for ex- 
ample, that his aircraft is 350 yards to right of the 
true course, and that the ground track of the air- 
plane is inclined at 6 degrees to the desired ground 
track. He can, therefore, give the pilot instructions 
to get on course, and he can also calculate the correct 
aircraft heading that should be taken after the air- 
plane is on course. 

Heading information is obtained by making use of 
a feature of the AN/APA-46 (Nosmo) attachment to 
the H2X radar (cf Section 8.3.2). The Nosmo at- 
tachment contains provision for determining the true 
ground track of the aircraft by use of the pulse 
doppler principle. In AN/APA-46 the direction of 
ground track of the aircraft is displayed on the PPI 
by an electronic cursor. In Micro-H Mark III, the 
electronic cursor was placed at right angles to the 
ground track. In order to fly a circular course about 
a beacon, the ground track direction of the aircraft 
at any instant must be tangent to the circle and hence 
perpendicular to the line joining the aircraft and 
beacon. On the PPI display, then, the right angle 
cursor should pass through the beacon response. The 
radar operator can tell not only if the aircraft is on 
the correct heading, but, if not, by how many degrees 
the heading should be corrected. 

Micro-H Mark III was not used in combat. Ac- 
curacy tests conducted by the Army Air Forces Board 
under simulated tactical conditions indicate a circu- 
lar probable error of 200 yd. 


RESTRICTS) 


H BOMBING 


113 



Figure 10. PPI photograph on a Micro-H Mark II training mission in an airplane near Nottingham, England. The 
aircraft is nearly on course and is approaching a check point. The beacon whose code is four evenly spaced signals is 
located at Beachy Head while the other beacon is at Winterton. 


10.2.4 Other Applications of the 
H Principle 

The Rebecca-Eureka beacon homing equipment 
has been modified for H operation, for use by RAF 
photo-reconnaissance aircraft. The Rebecca in its un- 
changed form is an airborne radar using fixed an- 
tennas and lobe switching, with a back-to-back echo 
matching display. The operator can tell when the 
aircraft is headed for the beacon by the fact that the 
signals are of equal strength in either lobe, and the 
echoes are matched in amplitude. In H operation, a 
single omnidirectional antenna is used, lobe switch- 


ing is abandoned and the two traces can be delayed 
independently in 10-mile steps up to 100 miles. Circu- 
lar courses are flown. To fly a circular course about 
one beacon at a range of 76.5 miles, and release a 
flash bomb at 59.3 miles from the other beacon, the 
operator views the first beacon on a 10-mile sweep de- 
layed 70 miles, and the other on the opposite trace, 
with the same expansion, delayed 50 miles. 

The system operates on any of five frequencies in 
the 214- to 236- megacycle-per-second band. The 
Eureka beacon, Mark II, weighs about 50 lb, and 
Mark III, about 17 lb. The beacons can readily be 
set up in a forward area and, once set up, can oper- 


i; i. >!'i ! k tkh p 



114 


BEACON BOMBING 


ate unattended. The British group who developed 
the Rebecca-H system estimate its flare-dropping 
accuracy as ±0.5 mile. 

Two applications of the H principle have been 
made to paratroop dropping. Here, the problem re- 
quires that portable beacons be sited close to the 
front lines, for use at relatively short ranges by troop 
carrier aircraft flying at altitudes below 1,000 ft. A 
variant of Micro-H was developed, using portable 
3-cm beacons (AN/UPN-3 preproduction models) 
and a modified AN/APS-10 radar. In principle the 
system is identical with Micro-H Mark II, except 
that the Norden bombsight is not used. Simulated 
tactical trials by the Army Air Forces Board indi- 
cated that paratroops could be dropped from an alti- 
tude of 500 ft with a circular probable error of 150 yd. 

The same scheme was developed independently by 
the 9th Troop Carrier Command in Europe, using 
the SCR-717 airborne radar and the AN/UPN-1 
portable, 10-centimeter beacons. Since the SCR-717 
does not contain a ranging unit, one was developed 
for the purpose. It uses a pulsed crystal oscillator, 
the oscillations being initiated by the modulator 
trigger. Interpolation between 1-mile pulses is made 
by phase shifting the sine wave oscillations with a 
condenser network. Beacons could be viewed on 
2- or 4-mile scope expansions, the delays in the 
sweeps being switched between sectors by a scanner 
mechanism. A synchronous tracking system was used 
for ranging on the beacon at variable range. In the 
latter sector, an air position indicator is used to move 
a phasing condenser so as to delay the sweep at such 
a rate that the beacon reply, and its reference range 
mark, were kept in the center of the sweep. Correc- 
tions can be made manually in the tracking rate 
which the air position indicator supplies. At the 
correct ranges, the warning light and the “jump” 
light are automatically lighted. 

The scheme was used only in demonstrations and 
on practice missions. On these occasions, it gave a 
probable error of about 150 yd. 

An interesting variation for extending the range of 
this device was developed by adding a ground position 
indicator [GPI]. The aircraft then flies at 1,500 ft, 
corresponding to an horizon range of 60 miles. An H 
course is flown to a point about 5 miles from the 
dropping zone. At the last precise H fix, the ground 
position indicator is started using that fix as its 
initial point. The aircraft then descends to 500 ft 
altitude, below the radar horizon, and the actual 
drop is made from the GPI data. The dropping ac- 


curacy of this scheme is estimated at 34 mile, if the 
length of the low-altitude run is 5 miles. 

10.2.5 Shoran 

The best-integrated and most accurate beacon 
bombing system used in World War II was Shoran. 79a 
However, its large-scale production came so late that 
its use was limited. Shoran is an H system operating 
in the 220 to 260 and the 290 to 320 me bands. Al- 
most alone of H systems, it is designed not as an at- 
tachment to or modification of some other equip- 
ment, but primarily as a beacon bombing system. In 
the Shoran scheme, the airborne equipment transmits 
on two frequencies, interrogating first one and then 
the other beacon. Both ground beacons reply on a 
third common frequency. 

Ground Stations 

The Shoran ground stations (AN/CPN-2) are un- 
coded beacons, weighing about 600 lb each, complete 
with power supply. (Double-pulse coding has been 
developed, but has not seen operational use. It in- 
creases the traffic handling capacity of the system by 
30 per cent.) A whole station has been flown into 
otherwise inaccessible territory with three or four 
trips of an L-5 (Piper Cub) aircraft. Recommended 
strength for a self-sufficient field unit complete with 
communications, transportation, maintenance per- 
sonnel, guards, and relief is 24 men, although three 
men can set up the AN/CPN-2 equipment and main- 
tain a continuous watch over it. The beacon may be 
set up with a nondirectional antenna or with a re- 
flector which confines the radiation pattern to about 
90 degrees. The two ground stations usually receive 
on frequencies separated by 20 me, and reply on a 
common frequency about 75 me from the middle 
frequency. 

The ground stations contain provisions for moni- 
toring and adjusting the frequency and the beacon 
delay time. In addition, a standard, temperature- 
controlled crystal, oscillating at 93,109 ± 2 cycles, 
produces pulses which are broadcast at intervals 
corresponding to exactly 100 statute miles. These 
pulses can be used to check the synchronization of the 
time base in the airborne equipment. 

Airborne Installation 

The airborne equipment (AN/APN-3 plus com- 
parator and K-l Computer), weighing approximately 
300 lb, consists of five units: the inverter power 


RESTRICTE] 


H BOMBING 


115 


supply, the transmitter, indicator, computer, and 
comparator. Of these, only the last three need be in 
the Shoran operator’s position. Receiving and trans- 
mitting antennas are separate, consisting of two non- 
directional dipoles, less than a foot long, one on top 
and the other beneath the aircraft. 

Indicator Unit. Ranging is done in the indicator 
unit. A crystal oscillator produces a sine wave at 
93,109 c, manufactured with a tolerance of plus or 
minus 5 c and adjustable to match in frequency the 
ground station crystals. Divider circuits produce 
from this sine wave, other sine waves Jfo an( ^ Moo of 
this frequency. Each of these three sine waves is fed 
into phase shifting inductance bridges, using pre- 
cision goniometers. The goniometers are geared to- 
gether in ratios of ten, so that a given phase shift on 
the 1-mile goniometer causes 3^o that phase shift on 
the 10-mile goniometer and 3^00 that phase shift on 
the 100-mile goniometer. Crossovers (where the sine 
wave voltage goes through zero) or peaks from the 
three phase-shifted sine waves are fed to a coinci- 
dence circuit, and the coincidence generates the pulse 
used for timing measurements. Thus, a range of 
86.937 miles corresponds to a phase shift of 313 de- 
grees on the 100-mile goniometer; 86 X 360 degrees 
plus 337 degrees on the 1-mile goniometer. The 
goniometer bridges are temperature compensated, 
and the 1-mile goniometer is accurate to within 1.5 
degree, or 22 ft. Separate phase-shifting networks are 
used for the two ground station signals. The total 
phase shift in each case, measured in miles, is indi- 
cated on a Veeder-Root counter having its smallest 
dial in the hundredths place, allowing estimates of 
thousandths. 

From the 931-cycle oscillation, a marker pulse is 
generated, the timing of which may be adjusted to 
compensate for system delays. Then, instead of hav- 
ing the beacon reply fall a variable time after this 
marker pulse, the novel method is employed of vary- 
ing the time when the transmitter is fired so as to 
make the reply coincide with the marker pulse. The 
marker pulse and the two beacon replies are dis- 
played on a circular trace, radial-deflection cathode- 
ray tube (J scope) on a 100-mile, 10-mile, or 1-mile 
time base as desired. Using the fastest time base, the 
sweep expansion is approximately 7 in. per mile. 
The trace is intensified only on the sweep in which 
the reply occurs. One of the two beacon reply pulses 
is inverted, so that with correct adjustment the 
leading edge of one beacon reply coincides with that 
of the marker pulse, while the leading edge of the 


other beacon reply is its mirror image beneath the 
trace. 

Computer. The Shoran system permits circular 
courses to be flown around either beacon. The bea- 
con which is held at constant range is known in 
Shoran parlance as the “drift station” and the one 
having variable range as thp “rate station.” The 
Shoran computer is so designed that one sets into it 
the ranges from the beacons to a point over the target 
(corrected for several precomputed factors), the bomb 
ballistics, the azimuth of the aircraft ground track at 
release, and the angle between the ground stations, 
measured over the target. While the pilot is guiding 
the aircraft on the bombing run by means of his 
instruments, the operator sets in the actual aircraft 
heading, and adjusts the computer “rate” mecha- 
nism so the beacon reply from the rate station remains 
lined up with the marker pulse. The computer then 
automatically determines the correct course and the 
release point (corrected for wind and ground speed) 
and releases the bombs automatically. 

The computer uses motor driven mechanical tri- 
angle solvers. These determine the cross-trail cor- 
rections and supply them to the indicator. An in- 
genious bridge circuit is used to maintain a motor 
driven shaft at constant but adjustable speed. This 
shaft drives the phase-shifting goniometer chain for 
the rate reply. Speed and displacement controls are 
used to align the rate reply with the marker pulse. 
Once these are correctly adjusted, the aircraft ground 
speed is effectively determined, and the computer 
multiplies ground speed by the time of bomb fall and 
subtracts the trail distance to determine the correct 
release point. Another Veedor-Root counter shows at 
any instant the miles to go to the release point. 

Comparator. Drift station information is presented 
to the pilot by means of the comparator. This circuit 
compares the phase of the received drift pulse with 
that of the marker pulse, and converts the difference 
into a small direct current which is read on the pilot’s 
direction indicator [PDI] meter thus telling the pilot 
whether he is to the right or left of the correct course. 
A differentiating circuit in the comparator shows by 
means of a second PD I needle whether he is increas- 
ing or decreasing his error. If both needles are cen- 
tered , the pilot is at the correct range and his heading 
is also correct. On the more sensitive of two ranges, 
the pilot sees full-scale deflection for a course error of 
400 ft; on the less sensitive range, full-scale deflection 
corresponds to a course error of 1,000 ft. 

Transmitter. The airborne transmitter is conven- 


RESTRICTED 


116 


BEACON BOMBING 


tional in design except that it may be switched 
rapidly between the two interrogating frequencies. 
The cycle of switching is of 34 o sec duration and is as 
follows: the transmitter broadcasts on the first fre- 
quency for 34o sec, is silent for 34o sec, broadcasts on 
the second frequency for 34o sec, and is silent for 
3^0 During the periods of silence, when the 
switching is accomplished, the transmitter is desyn- 
chronized from the time base, so that, on resumption 
of transmission, the timing of pulses is phased at 
random with respect to other airborne transmitters 
interrogating the same beacons. This scheme (identi- 
cal in principle with the Gee-H jitter circuit, de- 
scribed in 10.2.2) avoids the danger of having two air- 
craft continuously interrogate a beacon so nearly 
simultaneously that the beacon can reply to only one 
aircraft. 

Receiver. The Shoran airborne receiver has no un- 
usual design features; it is a sensitive low-distortion 
superheterodyne receiver. 

Use of Shoran 

First Shoran operations were carried out in the 
Mediterranean Theater by the 57th Bomb Wing of 
the 12th Air Force in the fall of 1944. In the spring of 
1945, the 42nd Bomb Wing of the 1st Tactical Air 
Force employed Shoran most successfully against 
targets in central France and southern Germany . 82a 
Both the 8th and 9th Air Forces had initiated pro- 
grams just before the close of the war in Europe, and 
at the close of the Pacific War the Far East Air Forces 
and the 20th Air Force had programs under way. 

Typical combat bombing accuracy results are those 
of the 42nd Bomb Wing, where 49 per cent of the 
bombs dropped from 12,000 ft in March 1945 fell 
within 400 ft of the target. In practice, a circular 
probable error of 180 ft is characteristic. Tests by the 
Army Air Forces Board at Orlando showed a reliable 
ability of the operator to locate himself in space 
within 50 ft. 

Advantages and Disadvantages of Shoran 

The major advantage Shoran has over other radar 
bombing devices is its accuracy. This is not so much 
because it is inherently more accurate than other 
bombing devices as it is because it is more nearly 
automatic and requires less operator training, thus 
minimizing the human link which is responsible for 
the bulk of the error in most other systems. The sys- 
tem has the advantage that a bombardier can be 
trained to operate the equipment in relatively little 


time, since the adjustments required are compara- 
tively simple. The pilot is required only to fly the 
PD I needle/' ’ Pilot and bombardier are trained to- 
gether as a team. Shoran has, in common with 
Gee-H and Rebecca-H, the advantage that low-drag 
antennas are used on the aircraft. 

Shoran has the disadvantage that it is heavier and 
bulkier than other H systems and does not combine 
other features in the same equipment, such as radar 
bombing or long-range navigation. 

10.3 OBOE BEACON BOMBING 
SYSTEMS 

In Oboe systems, ranging is done by two ground 
stations. This results in two features characteristic of 
such systems. First, a means is provided for com- 
municating from the ground stations to the bombing 
aircraft; and second, the ground radar stations may 
be as elaborate as desired (in practice a typical ground 
station has a personnel complement of 40, and the 
apparatus fills several rooms). Three different types 
of Oboe have been used, Oboe Mark I, II, and III. 

10.3.1 Oboe Mark I 

This system operates in the 211- to 236-mc band. 
In the earliest form, the airborne beacon is interro- 
gated and replies on the same frequency. In later 
versions, as an antijamming measure, the ground sta- 
tion transmits two simultaneous pulses on different 
frequencies, and the airborne beacon replies on a 
single frequency if this coincident interrogation is 
received. 

Ground Stations 

Oboe Mark I Ground Stations are high-power, 
directional radars, similar in transmitting and re- 
ceiving equipment to the coastal early-warning radars 
from which they were derived. Since the ground sta- 
tion may be used either as a “cat” station (about 
which the aircraft flies at constant range) or as a 
“ mouse”’ station (which gives bomb release informa- 
tion), two types of computers are required. In either 
case, the aircraft reply is displayed on each of two 
large A scopes. The first of these may have either a 
250-mile or a 9-mile sweep, on which 1- and 10-mile 
calibration marks are also displayed. On the second 
A scope, any desired portion of the sweep may be 
presented together with calibration marks on a scale 
which can be expanded to 534 in. per mile. 

If the station is to function as a cat, a “ double 


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OBOE BEACON BOMBING SYSTEMS 


117 


strobe” is placed at the desired aircraft range (see 
Figure 11). This double strobe consists of two 4-//sec 
segments of the sweep, separated by 1 //sec. On the 

A CAT STATION 

AIRCRAFT 

SIGNAL 


JL -A. - 

CALIBRATION !<- STR0BE -! I^TROBE*! 
PULSE 1 



stant the signal reaches the middle pip, the operator 
presses a second button which reverses the clock. If 
the aircraft speed remained constant, the aircraft 
would just be at the corrected target range (the last 
blackout pip) at the instant the clock returns to zero. 
Actually the automatic mouse transmits a release 
signal at a time earlier than this by the time of bomb 
fall. The function of the automatic mouse, then, is to 
provide a warning signal and a bomb release signal, 
using the aircraft ground speed determined on the 
last portion of the bombing run. 


B MOUSE STATION 

CALIBRATION PULSES 

w 

/ I I I I ' 

AIRCRAFT I 1 

RETURN BLACKOUT 

PULSES 

Figure 11 . Oboe ground displays. 

trace, the strobes appear as blacked-out portions. The 
center of the l-/zsec portion is at the correct range. 
The leading edge of the aircraft reply (receiver video 
pulse) is differentiated, and a symmetric signal is 
derived from this leading edge. If the aircraft is at 
exactly the right range, the peak of the signal comes 
over the l-/zsec portion, and the edges of the signal 
overlap the two strobes equally. If the aircraft devi- 
ates from course, the signal overlaps one strobe more 
than the other. Voltages are derived in the circuit 
which are proportional to the overlap of the signal 
with either strobe. These voltages are used to modu- 
late the signal transmitted to the aircraft pilot to 
indicate to him whether or not he is flying the correct 
course. The system is sensitive enough so that 10 per 
cent modulation is introduced when the aircraft is 
17 yd off course. 

If the station is to operate as a mouse station, a 
series of equally spaced blackout pips, each being 0.5 
//sec wide (see Figure 11), is so placed that the last 
one occurs at the release range (corrected for beacon 
delays and trail distance of bomb). When the air- 
borne beacon signal reaches the first of these pips, 
the ground radar operator presses a button which 
starts a clock running. (Both mechanical and elec- 
tronic timing devices have been used.) At the in- 


Airborne Equipment, Oboe Mark 1 

The airborne equipment consists of a receiver, con- 
trol box, transmitter, and a filter which decodes in- 
telligence from the ground stations (see Figure 12). 
This information is then presented to the pilot and 
bombardier by interphone. The receiver, control 
box, and modulator-transmitter contain no unusual 
features, and the filter will be described in the dis- 

RECEIVING PILOT'S 



Figure 12. Oboe airborne equipment. 


cussion of the communications system. The an- 
tennas are fixed, the antenna patterns being essen- 
tially nondirectional in the horizontal plane. 

Communications System 

In Oboe Mark I, a scheme for varying the time 
interval between successive interrogating pulses is 
used for communication. This scheme is known as 
space modulation. 

The cat ground station transmits a pulse every 3^33 
second. Between each pair of these fixed pulses is 
transmitted a modulated pulse, the timing of which 
can be varied from half to three-quarters of the in- 
terval between fixed pulses (see Figure 13). In the 
aircraft, one output from the receiver is applied to 
the first grid of a peaked audio amplifier, called the 
filter, and resonant at 266 c. If the modulated pulse 
is phased at the half-way position, the filter is “rung” 
at its resonant frequency, and maximum output 
amplitude is obtained. If the pulse is phased at the 






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BEACON BOMBING 


three-quarters position, it is in opposition to the 
preceding pulse, and no filter output is obtained. In 
practice, the equisignal is obtained by phasing the 



FIXED MODULATED FIXED 

PULSE PULSE PULSE 


►-LIMITS OF-*H 
MOTION OF j 
MODULATED I 
PULSE ' 


532 


SEC 


Figure 13. Space modulation communication system. 


each frequency, and the receiver outputs are fed to 
a coincidence tube. Unless the aircraft is interro- 
gated simultaneously on the two frequencies, the air- 
borne beacon does not reply. 

In Oboe Mark I-K with “ latching/ ’ two frequen- 
cies are again used in the ground to air transmission. 
A delay of the order of 10 /xsec is introduced between 
the pulses on the two frequencies. A compensating 
delay line is placed in the output of the appropriate 
airborne receiver, and the delayed signal is passed to 
a coincidence tube as before. The aircraft then re- 
plies only to paired pulses on the correct two fre- 
quencies if they are separated by the correct time 
interval. 

In both the K and the K-latching systems, the air- 
borne beacon replies on one frequency only. 


modulated pulse at the five-eighths position, and the 
depth of modulation is determined by the range, 
centered on this position, through which the pulse 
moves. 

The pilot “ flies a dot-dash beam” ; that is, he hears 
a succession of dots in the earphones if he is too close 
to the cat station, and a succession of dashes if he is 
too far. As the correct range is approached, the dots 
and dashes merge together, and, at exactly the correct 
range, a steady tone is heard. This is done at the 
ground station by feeding the output of an unbal- 
anced multivibrator (which produces the dots and 
dashes) and the output voltage from the double 
strobe (see “Ground Stations” in Section 10.3.1) 
into appropriate circuits. If the aircraft is on the far 
side of the course, the voltage output from these cir- 
cuits is greater during the dash (long) part of the 
multivibrator cycle, and less during the dot part. This 
voltage is used to determine the delay of a phanta- 
stron which initiates the variable pulse. 

Communication from the mouse ground station is 
provided by using a constant pulse repetition fre- 
quency of 194 pulses per second. This “rings” a sec- 
ond peaked audio amplifier in the airborne filter. 
Signals of the Morse type are sent to the bombardier 
by interrupting transmission, either by a hand key or 
by automatic keying by the “mechanical mouse” 
which gives the warning and release signals. 

Antijamming Modifications 

Two modifications of Oboe Mark I were introduced 
as antijamming measures. In Oboe Mark I-K the 
ground stations transmit simultaneous pulses on two 
frequencies. The aircraft carries two receivers, one for 


Use of Oboe Mark I 

Oboe Mark I, which was developed by the British 
Telecommunications Research Establishment, was 
used by the Pathfinder Force (RAF 8 Group) of the 
Bomber Command, Royal Air Force. The scheme was 
employed primarily for night flare-marking opera- 
tions, in which the target was bombed with marking 
flares by several Oboe-equipped Mosquito aircraft, 
and the flares were then bombed by a main force of 
perhaps 500 heavy bombers. The first Oboe operation 
was in December 1942 and Oboe Mark 1 was used ex- 
tensively through 1943 and with other models of 
Oboe to the end of the European phase of World 
War II. Fifty operations had been held by June 1943 
and the Ruhr industrial complex was largely para- 
lyzed by Oboe raids. 

Oboe Mark I operational accuracy, when used with 
bombs (rather than flares) at an altitude of 30,000 
feet, is about 34 mile circular probable error. 

10.3.2 Oboe Mark II 

Oboe Mark II is the microwave version of Oboe 
Mark I. The transmitting and receiving equipment is 
in the 3,150 to 3,240 me band, while the ranging and 
communications systems are unchanged in principle. 

Ground Stations 

Several types of Oboe Mark II ground stations 
were developed. In the fixed (permanently installed) 
ground stations, the equipment is located in build- 
ings. Modified ASG 10-cm transmitters and modula- 
tors were used, delivering 30 kw pulse power into 


B 


RESTRP 


OBOE BEACON BOMBING SYSTEMS 


119 



Figure 14. Bomb strike photograph showing the destruction of a bridge in Northern France by airplane equipped 
with Oboe Mark II. (U. S. Army.) 


highly directional parabolic reflectors. Tunable mag- 
netrons were installed and the transmitting frequency 
was constantly monitored to insure transmission on 
the assigned frequency. 

In the mobile ground stations, the same equipment 
is mounted in two vans. A third van carries power 
equipment, and the three vans with associated vehi- 
cles make up a convoy which can be moved into a site 
and put into operation as soon as the necessary com- 
munications links are established. 

Oboe Mark II Equipment 

The airborne equipment for Oboe Mark II consists 
of a modified ASG airborne radar transmitter-modu- 
lator, a receiver of British manufacture, and the same 
type of filter as used in Oboe Mark I. 

The antenna is a vertical dipole array, giving a 
beamwidth of 15 degrees in the vertical plane and 60 
degrees in the horizontal plane. The antenna can be 
rotated to the correct bearing to receive the ground 
station. 


Use of Oboe Mark II 

Oboe Mark II was developed jointly by the British 
Telecommunications Research Establishment and 
the Radiation Laboratory. It was felt that the con- 
version to 10-cm frequencies made the system sub- 
stantially more secure than Mark I. Oboe Mark II 
was used by 8 Group (Pathfinders) of the Bomber 
Gommand, Royal Air Force from February 1944, to 
the close of the war. The 9th Bomber Command, 
USAAF, also equipped one Pathfinder Squadron 
with Oboe Mark II. The American group had its first 
Oboe operation in March 1944, and used the system 
extensively from that time to the end of the European 
phase of World War II. 

Typical combat accuracy figures are those for the 
9th Bomber Command in June and July 1944, giving 
a circular probable error of 700 ft. Figure 14 is a 
strike photograph taken from an airplane flying under 
Oboe Mark II control. For practice bombing, the 
circular probable error is about 450 ft. 


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BEACON BOMBING 


10.3.3 Oboe Mark III 

Oboe Mark III is an attempt to increase Oboe 
traffic handling capacity without increasing the num- 
ber of radio frequency channels. This is done by using 
several different pulse recurrence frequencies, each 
furnishing one communication link, on the same radio 
frequency. As originally conceived, each ground sta- 
tion w^ould have one transmitter and one receiver, 
and would have separate displays and computing 
equipment for each aircraft controlled. One ground 
station transmitter would then control several air- 
craft simultaneously. As used operationally, the 
spread in airborne frequencies was such that it was 
considered better to have one ground station trans- 
mitter and receiver, as well as displays, and com- 
puters, for each communication link desired, but 
several ground stations could operate simultaneously 
on the same radio frequency. Oboe Mark III oper- 
ates on the same radio frequency (in the 10-cm band) 
as Oboe Mark II, and the transmitting and receiving 
equipment is identical in the two systems. 

Communications System 

In Oboe Mark III, each ground radar station is 
assigned a characteristic pulse recurrence frequency 
(97.5 pulses per second is a representative value) and 
no movable pulses are used. Intelligence is conveyed 
by modulating the duration of the interrogating pulse 
between 2 and 4 nsec. Thus, a succession of 2-jusec 
pulses produces no audio tone in the filter output and 
a succession of 4-//sec pulses produces maximum 
audio output. A dot-dash modulation is obtained by 
using a voltage (produced in the same manner as the 
voltage wffiich phases the movable pulse in Mark I 
and II) to vary the duration of the pulse between 
these limits, the “equisignai” being a succession of 
3-/xsec pulses. 

Airborne Equipment 

The airborne equipment differs from that of Oboe 
Mark II only in the use of a new type of filter. This 
filter must accept only pulses which occur at the cor- 
rect pulse repetition frequency and must demodulate 
these pulses to provide audio signals for the pilot 
and bombardier. 

Selection of the correct frequency is accomplished 
by use of a phantastron (delay circuit) chain. In the 
example given above of 97.5 pulses per second, the 
interval between pulses is 10,257 nsec. Two phan- 


tastrons in series (so that each phantastron recovers 
during the half cycle when the other is running) pro- 
duce a blanking pulse which deadens the receiver for 
10,205 nsec. The end of this pulse starts another 
phantastron which runs for 300 nsec or until a signal 
appears from the receiver, whichever occurs first. 
While the latter phantastron is running, the receiver 
is sensitive. The end of the pulse from this phan- 
tastron starts the first phantastron again, so that 
in the absence of a signal the phantastron cycle is 
completely free-running. 

If a signal occurs within the period of sensitivity of 
the receiver, this signal is accepted. Accepting the 
signal starts the blanking phantastrons, and the re- 
ceiver is not again sensitive until about 50 /xsec before 
the next signal from the same ground station is ex- 
pected. When that signal arrives, the process is re- 
peated, so the filter “locks on” to the ground station 
transmission, accepting all pulses. If, however, the 
signal initially occurs during the “dead time” of the 
receiver, the sensitive time of the receiver is increased 
250 nsec per cycle, and within Y sec the filter cycle 
will have advanced in phase sufficiently to accept a 
signal and thus “lock on” to the ground station 
transmission. It is thus possible to accept a ground 
station transmitting at 97.5 pulses per sec, and re- 
ject one transmitting at 99 pulses per sec. 

The filter removes the first 2 jusec wddth from the 
accepted pulses and derives a d-c voltage which is 
proportional to the remaining duration of the pulse. 
This voltage determines the gain of an audio ampli- 
fier which amplifies a constant audio frequency 
signal, and the amplifier output is supplied to ear- 
phones. Separate channels in the filter are used for 
the cat and mouse stations, the audio outputs going 
to the pilot and bombardier respectively. 

Use of Oboe Mark III 

Oboe Mark III first saw operational use in the 
summer of 1944, and was gradually replacing Oboe 
Mark II at the close of the European bombing oper- 
ations. In addition to increasing the traffic handling 
capacity of the centimeter Oboe system, the Oboe 
Mark III is more secure than Oboe Mark II, because 
jamming pulses, to be effective, must not only be on 
the correct radio frequency but also on the correct 
pulse recurrence frequency. Oboe Mark III was used 
by the RAF and by the 9th Bomber Command. 

Accuracy of the Oboe Mark III system, as would 
be expected, is the same as that of Oboe Mark II. 


jm 


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O- 


BEACON OFFSET BOMBING 


121 


10.4 BEACON OFFSET BOMBING 

10.4.1 Beacon Offset Bombing 

Technique 

In beacon offset bombing, portable beacons are 
placed near the target as reference points for offset 
bombing by radar equipped aircraft. Techniques 
have been developed for the use of this scheme at low 
(100 to 500 ft) and medium (8,000 to 12,000 ft) alti- 
tudes. The usual procedure is to fly over the beacon 
on the beacon-to-target ground track and release 
bombs at a time after passing the beacon such that 
the bombs strike at the target distance from the 
beacon. 

Several equipment combinations are possible. In 
the 160 to 234 me range, the following may be used: 

1. Rebecca (AN/APN-12 or AN/APN-5) as air- 
borne equipment with Eureka beacons (AN/PPN-1, 
AN/PPN-2, and AN/TPN-1). 

2. SCR-729 airborne systems with modified SCR- 
695 beacons. 

3. SCR-540 airborne systems with modified SCR 
695 beacons. 

In the microwave region (3,000 to 10,000 me), the 
following combinations can be used: 

4. BUPS (AN/UPN-1 or AN/UPN-2) beacons 
with SCR-717, SCR-720, or AN/APS-2 airborne 
radar systems. 

5. BUPX (AN/UPN-3 and AN/UPN-4) beacons 
with AN/APQ-13, AN/APS-15, AN/APS-15A, 
AN/APS-10, or AN/APS-4 radar systems. 

10.4.2 Low Frequency Systems 

Rebecca is an airborne interrogator-receiver, using 
fixed antennas on either side of the aircraft, and an 
echo matching presentation to determine when the 
beacon is dead ahead. Rebecca transmits on any one 
of five preselected radio frequencies in the 214 to 234 
me band, and receives on twm preselected and ad- 
jacent radio frequency channels. Eureka is an ultra- 
portable transponder beacon with five receiver and five 
transmitter channels chosen to work with Rebecca. 
The other combinations work together as do Rebecca- 
Eureka. The SCR-729, for example, operates in the 
157 to 186 me band and the SCR-695 (an airborne 
beacon in its unmodified form) acts as the ground 
beacon. 

Advantages of Low-Frequency Systems 

The ground beacon is highly portable. The Eureka 
beacon complete with battery and mast weighs 32 lb, 


and can conveniently be carried by paratroopers 
when jumping. (Comparable microwave beacons are 
twice as heavy.) Because of greater low-frequency 
diffraction around obstacles, siting restrictions are 
less severe than for microwave beacons. A Eureka 
beacon set up below tree level in wooded territory 
can be seen by a Rebecca (altitude 8,090 ft) at 28 
miles. Comparable microwave performance in such 
terrain yields ranges of 6 to 15 miles. 

One man can set up a Eureka and put it in opera- 
tion in 60 seconds. 

Disadvantages of Low-Frequency Systems 

The primary argument against low-frequency sys- 
tems is the poor azimuth discrimination when com- 
pared with the microwave systems. If the ground 
range from the beacon to the aircraft is much less 
than the aircraft altitude, satisfactory azimuth indi- 
cation cannot be obtained. Eurekas on a given fre- 
quency cannot be placed closer to each other than 3 
or 4 miles even for low-altitude bombing, if they are 
to be resolved in azimuth. The siting of a number of 
such beacons along a front line for the dual purposes 
of providing radar bombing reference points and de- 
marking the front line is therefore not feasible. 

10.4.3 Microwave Systems 

The BUPX’s are lightweight radar beacons in the 
10,000-mc region, designed to work with radars of 
the H2X type. The beacon reply is range coded to 
identify the particular beacon. The BUPS’s are 
lightweight radar beacons in the 3,000-mc band, op- 
erating with search radars in that frequency range. 
Of the two combinations, the H2X-BUPX scheme is 
to be preferred, because the H2X radars are designed 
for bombing as well as for search, and because of the 
greater azimuth resolution possible at the higher fre- 
quency. 

The advantages of microwave systems can be in- 
ferred from the previous discussion. The increased 
azimuth resolution permits BUPX beacons to be 
spaced as close together as 34 mile and still be re- 
solved at a distance of 6 miles, with the result that 
the number of targets for which individual reference 
points can be set up in a given area is much increased. 
One can read the bearing of the beacon from the air- 
craft, not merely tell whether or not it is dead ahead. 
Medium- and high-altitude bombing becomes pos- 
sible, and offset distances can be increased for a 
given desired accuracy. Use of a number of reference 


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122 


BEACON BOMBING 


beacons for front line marking is practical. The sys- 
tem gains in flexibility, since the H2X radar can be 
used for unaided radar bombing if desired, while all 
the low-frequency systems require that the beacons 
be in operation for a successful mission. The micro- 
wave beacons can be seen by all radars with which 
they are designed to work, whereas a Rebecca-Eureka 
pair must be tuned for each other on preassigned fre- 
quencies. 


volved the use of check points at known slant ranges 
from the beacon to synchronize the Norden bomb- 
sight. Results from 10,000 ft, using an offset distance 
of 2 miles gave bombing errors of approximately 
2,000 ft. 

The fact that a beacon gives a clear-cut signal, 
positively identified, suggests the possible future de- 
velopment of beacon offset bombing using an offset 
bombing computer. 


io.t.4 Tactical Use 

Although beacon offset bombing did not see oper- 
ational use in World War II, tactical trials were con- 
ducted by the AAF Board to develop methods of use 
and to determine accuracy. For low-altitude work, a 
technique was developed in which the first step was 
to assign the altitude, airspeed, and direction of ap- 
proach to target for the bombing aircraft. The beacon 
was then located at the point such that if the aircraft, 
flying as briefed, released bombs at the instant it 
passed over the beacon, those bombs would strike the 
target. (For example, an aircraft flying at 100 ft 
altitude with a speed of 200 mph would require a 
beacon located 270 yd from target.) The air crew can 
determine the instant of passing the beacon, with the 
microwave system, by the fact that the beacon reply 
becomes, for a fraction of a second, a series of circles 
about the center of the display. With Rebecca- 
Eureka, the ground-beacon operator keys the beacon 
off the air at the instant the aircraft is overhead. The 
Rebecca-Eureka system, for an offset distance of 800 
ft and a bombing altitude of 100 ft, gave as a typical 
result an azimuth error of 135 ft and a range error 
of 45 ft. The H2X-BUPX combination, for an offset 
distance of 2500 ft and a bombing altitude of 200 ft, 
gave an average radial error of 300 ft. 

Medium- and high-altitude bombing is accom- 
plished with techniques nearly identical with radar 
offset bombing. In one scheme, evaluated by the AAF 
Board, the aircraft flies toward the beacon on a 
briefed heading, the heading being corrected for drift 
angle so that the extended ground track of the air- 
craft passes through the beacon and the target. The 
bombing problem is set up as though the aircraft 
were to bomb the beacon. At the instant the beacon 
reply touches the bombing range circle, a stopwatch 
is started. The aircraft holds its airspeed and heading 
for the number of seconds required to fly from beacon 
to target, and then bombs are released. 

Another method evaluated by the AAF Board in- 


10.5 HYPERBOLIC NAVIGATION 
SYSTEMS 

10.5.1 Introduction 

Hyperbolic navigation systems, 55 such as British 
Gee and American Loran, were not originally in- 
tended for blind bombing use. Their normal function 
is to provide an unlimited number of fairly accurate 
fixes for navigation over a large area, rather than the 
few very precise fixes required for blind bombing. 
The lattice or grid formed by the crossed position 
lines has, however, very appealing similarities to the 
cat-mouse course often flown with the various beacon 
systems (see Section 10.2.1). 

These similarities suggest the technique of flying 
down one position line by keeping constant the ob- 
served time difference of one pair of pulses. The ob- 
served time difference of the other two pulses changes 
as the aircraft traverses the ground track defined by 
the first position line, and the bombs are released 
manually when this second time difference has 
reached the correct value. 

In area bombing, with a hyperbolic navigation 
system, appropriate standard navigation charts are 
used, which are already overprinted with the com- 
puted lines of position. This has the advantage that 
no additional calculations need be made for an indi- 
vidual mission. A further advantage is that both the 
navigation to the target area and the release of bombs 
can be handled by the use of a single piece of appa- 
ratus and a single basic technique. 

The obvious disadvantage of blind bombing with 
hyperbolic navigation devices is that the fix errors 
may be ten or more times those of beacon-ranging 
systems. This makes unprofitable any type of offen- 
sive bombing where the target is not distributed over 
a considerable area. Because of this fix inaccuracy, it 
is seldom worth while to apply any ballistic correc- 
tions other than an adjustment of the mouse reading 
to compensate for the forward travel of the bomb. 


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. , 


HYPERBOLIC NAVIGATION SYSTEMS 


123 


10.5.2 Navigation and Blind 
Bombing with Gee 

Ground Station Arrangement 

The basic Gee ground station arrangement consists 
of three stations spaced at intervals of approximately 
70 miles, the line connecting them making an oblique 
angle of between 120 and 150 degrees. The center sta- 
tion is known as the Master , or A station, while those 
on either side are known as Slaves, designated B and 
C respectively. All three stations operate on the same 
radio carrier frequency, which is in the range between 
30 and 80 me. The Master emits short pulses at the 
rate of 500 per sec, every other pulse being marked 
by a slightly delayed, blinking, identification pulse 
known as the “ ghost ’ 1 pulse. The B Slave emits 
pulses at 250 per sec, the delay of each pulse behind 
the unmarked Master pulse being precisely main- 
tained. The C Slave also transmits at 250 pulses 
per sec, each pulse occurring at a time shortly 
following the transmission of the Master’s ghost- 
identified pulse. The time sequence of transmission 
is thus A-B-A (ghost)-C, indefinitely repeated. The 
timing delays and the distances between the stations 
are so chosen that the pulses are always received in 
this same order no matter where the receiver may be 
located. Only the A (ghost) pulse has a distinctive 
appearance, but the reception sequence provides 
positive identification of all pulses. 

Airborne Instrumentation 

By the end of World War II the Gee Mark II air- 
borne set (A.R.L 5083) was carried in nearly every 
Allied aircraft with a crew of two or more. For ob- 
serving and measuring the time delays with the 
necessary accuracy, two basic presentations are used, 
giving essentially “coarse” and “fine” magnifica- 
tion. Figure 15 shows the appearance of the scope 
during the various manipulations. A single 5-in. 
cathode-ray tube is employed with linear horizontal 
sweeps and with signals appearing as vertical deflec- 
tions, as in an A scope. The complete picture is 
traced out in 4,000 jusec; on the slow-sweeping “Main 
Time Base” this period is divided into two equal 
intervals, each of which is presented as a horizontal 
sweep with fast flyback. The two traces are separated 
vertically by a space of about 1 in. By manipulating 
the frequency control of the 150-kc crystal oscillator, 
an operator first places the two master pulses near the 
extreme left side of the two sweeps. The ghost- 
identified pulse is placed on the lower sweep and the 




MAIN TIME BASE STROBE TIME BASE (EXPANDED SWEEP) 

CAL PIP DISPLAY CAL PIP DISPLAY 

B = 1 1 C= 33 B = 1 1 .38 C® 33.73 

Figure 15. Gee system indication. 

other master pulse then appears above it on the upper 
sweep. The B and C slave pulses appear to the right 
of the master pulses, on the upper and lower traces, 
respectively. A coarse setting of the two expand ed- 
sweep (“strobe”) delays is then made; when the 
strobe is properly placed around its slave signal the 
slave signal is inverted and appears as a dowmward 
deflection (see Figure 15 A). 

The accurate setting and measurement is made 
with expanded sweeps of approximately 90 gsec dura- 
tion. The picture consists of four such sweeps, one for 
each pulse, displaced vertically in two sets- (Figure 
15B). In each set the upper trace displays the master 
signal ( A ) with an upward deflection, while the sec- 
ond trace appears about 0.2 in. lower and displays 
the slave signal (B), with a downward deflection. 
The delay control is manipulated to place the slave 
pulse directly under the master signal, and when the 
same has been done for the other set of pulses the 
apparatus has stored the readings required for a fix. 
The two delay measurements are then made, by 
means of calibration pips derived from a 150-kc os- 
cillator and subsequent dividing stages. The result is 
two four-figure numbers. The navigator is furnished 
with charts which bear overprinted lines correspond- 
ing to the first three figures of each number. Inter- 
polation gives the two position lines whose inter- 
section gives the fix. 

Blind Bombing with Gee 

It is evident that the Gee indicator is quite well 
adapted to flying the hyperbolic equivalent of a cat- 


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124 


BEACON BOMBING 


mouse course, since one of the two delay readings is 
then kept constant. Position line curvature is not 
troublesome when the bombing run does not ap- 
proach closer to a ground station than approximately 
30 miles. The basic technique for flying the hyper- 
bolic course with Gee is essentially the same as for 
flying a circular course with Gee-H. The principal 
difference is that, with Gee, the operator continues 
to make slight adjustments of the oscillator dial in 
order to keep the received pulses on the strobes. 
As in Gee-H and Micro-H Mark I, the operator gives 
the pilot verbal instructions such that proper course 
and proper heading will be achieved simultaneously. 
This process requires much skill and judgment and 
is a source of operational error. 

Since Gee uses radio frequencies in excess of 30 me, 
the reliable range for a particular altitude is only a 
few tens of miles greater than the line of sight dis- 
tances from the ground station. The Gee service area 
is thus not much greater than that of the more ac- 
curate beacon ranging systems, and Gee has not been 
used extensively for blind bombing. It has, however, 
been used for dropping paratroops and has seen a 
great deal of service in RAF flare-dropping path- 
finder operations. 

In this latter technique, pathfinding aircraft navi- 
gate to the target area with Gee and release there a 
series of parachute flares. A master controller (so- 
called “ Master Bomber”) in a fast low-flying aircraft 
locates the particular flare most suitable as an aim- 
ing point and has it reinforced with other, distinctive, 
flares. The main bomber force then bombs visually on 
this flare. Much use w*as made of this technique bj^ 
the Royal Air Force. 



Although the accuracy of Gee becomes consider- 
ably less than that of beacon ranging systems at dis- 
tances of more than 50 miles from the ground sta- 


tions, its accuracy at shorter distances is quite com- 
parable. This can be and has been a considerable dis- 
advantage in that enemy aircraft carrying captured 
equipment are given a very accurate means of bomb- 
ing the territory near the ground stations. This fea- 
ture was exploited by the Germans in raids on 
London during the last weeks of February 1944. 
Rather alarming accuracy was obtained until the 
situation was recognized and the Gee stations tem- 
porarily removed from service. 

10.5.3 Navigation and Blind 
Bombing with SS Loran 

Differences between SS Loran and Gee 
In comparing SS Loran and Gee, with reference to 
blind bombing use, SS Loran is found to differ in two 
major respects, one of which is somewhat of a disad- 
vantage while the other is of very considerable ad- 
vantage. SS Loran stations operate in pairs instead 
of triplets and the SS Loran airborne indicator 
(AN/APN-4) is adapted to making but one delay 
reading at a time. This means that, in a fast-flying 
aircraft, accurate running fixes must be made, which 
requires considerable skill. On the other hand, the 
radio frequency of approximately 2 me makes pos- 
sible a greatly extended usable range and a novel ar- 
rangement of ground stations, since, at night, iono- 
spheric reflections may be used. In traveling between 
two separated points on the earth’s surface these sky 
waves require somewhat more than 60 jusec longer 
than would the direct ground wave. Both the average 
value of this sky-wave delay and the probable devi- 
ation of a single measurement from this value have 
been experimentally determined as a function of 
range. The accuracy and constancy are sufficient to 
make one-hop sky waves very useful for synchroniza- 
tion and navigation. Since the received signals have 
been reflected from above, good reception is obtained 
at all altitudes down to the lowest, an advantage not 
found in any other radio navigation or blind bomb- 
ing system. The height of the reflecting layer is suffi- 
ciently great so that the stations of a pair may be 
separated by as much as 1,200 nautical miles and 
still be synchronized at nighttime over a single-reflec- 
tion transmission path. The area which is served by 
single-hop reception from a pair of ground stations 
lies between them and is bounded as shown in Fig- 
ure 16. By suitably locating another pair of similar 
stations, fixes can be obtained over an extremely 
large area: as much as 1,000,000 square miles. Al- 


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NONBOMBING APPLICATIONS OF PRECISION BEACON NAVIGATION 


125 


though the timing accuracy depends on the vagaries 
of ionospheric transmission, crossing angles are fav- 
orable over most of the service area and the position 
lines do not diverge seriously. The net result is that 
on the average one half of the fixes obtained will be 
in error by less than a mile, an accuracy which is 
substantially uniform over the service area. An im- 
portant advantage is that since navigation is less ac- 
curate near the stations than over enemy territory, 
captured equipment will not be as useful to the 
enemy as was the case with Gee. 

Airborne Instrumentation 

The signals from different pairs of SS Loran ground 
station pairs are radiated at the same carrier fre- 
quency and are distinguished from one another by 
small precise differences in recurrence frequency. On 
each recurrence rate there are thus but two signals, 
one from each station of the synchronized pair. The 
airborne indicator is provided with a switch for mak- 
ing the signals of the desired pair stand still on the 
oscilloscope sweep while those of other pairs move at 
velocity sufficient to avoid interference. In other re- 
spects, the method of setting and measuring the 
received time differences is somewhat similar to that 
used in the Gee indicator, except that on the ex- 
panded presentation two sweeps are used instead of 
four. Only one time-difference measurement can be 
made at a time, and since a 100-kc crystal is used, the 
measurements are estimated to the nearest micro- 
second. To make the other delay measurement, on 
the second pair of stations, repetition of the three 
part measuring process is necessary, requiring be- 
tween 1 and 2 minutes. This time has been reduced 
substantially by the addition of a switch and a dupli- 
cate set of delay controls so that in making a series 
of alternate observations on the two rates only small 
delay readjustments are needed. This modification 
was made and used by the RAF. 

Blind Bombing with SS Loran 

During the early months of 1944, four ground sta- 
tions were installed by the Royal Air Force, aided by 
Radiation Laboratory personnel. The stations of one 
pair were located in Scotland and Tunisia respec- 
tively, while those of the other pair were located in 
Algeria and Libya. These four stations produced 
coverage over most of Europe as shown in Figure 17. 
Although the stations were ready several months 
earlier, it was not until September that they were 
placed in operation. At this time the RAF Bomber 


Command made immediate and large-scale use of the 
system and continued to do so until the end of the 
war. For a considerable period the SS Loran system 
provided the only means of navigation on raids deep 
into Germany and occupied Europe. 

The Royal Air Force was the largest user of 
SS Loran, since the nighttime hours, when the sky- 
wave signals were useful, coincided with their period 
of most active operation. By March 1945, approxi- 
mately 800 aircraft were fitted with Loran. In gen- 
eral, both Gee and Loran were installed but in some 
aircraft one set of mountings was used interchange- 
ably for carrying either Gee or SS Loran, the choice 
depending on the nature of the mission. One group 
of the RAF made over 8,000 sorties with Loran- 
equipped aircraft and it is believed that on one-third 
to one-half of these sorties Loran was used for navi- 
gation. Gee was universally used for navigation in 
the region of the home airfields. In raids which were 
conducted on targets within Gee coverage, the use of 
Gee was preferred because of the greater ease of 
operation. 

RAF Mosquito airplanes used SS Loran for several 
months on almost nightly nuisance raids over Berlin. 
On these raids both the navigation and bomb release 
were done with SS Loran. A total of over 13,000 
sorties were made by RAF SS Loran-equipped air- 
craft. This, therefore, is the major example of the use 
of a hyperbolic navigation system for blind bombing 
on a target area. 

10.6 NONBOMBING APPLICATIONS OF 
PRECISION BEACON NAVIGATION 

10.6.1 Paratroop Dropping 

Two nonbombing problems for which beacon 
bombing techniques are useful are paratroop drop- 
ping and photo-reconnaissance. The most widely used 
blind navigational method for troop carrier work in 
World War II was Rebecca-Eureka dropping zone 
location. The methods employed were entirely simi- 
lar to those described in the section on beacon offset 
bombing (Section 10.4.1). Combat accuracy has been 
estimated from one-quarter to one mile average error 
in locating the center of a stick of paratroops. Oper- 
ational use has also been made of Gee {not Gee-H) 
for dropping paratroops, but some Troop Carrier 
operations personnel have stated that Gee is not 
sufficiently accurate for locating the actual dropping 
zone although it is very useful in navigating to the 
right general area. 


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126 


BEACON BOMBING 



10.6.2 Aerial Reconnaissance 
Applications 

Several of the schemes previously described in this 
chapter have been adapted for use in aerial recon- 
naissance or mapping, and in principle any ground- 
controlled blind bombing scheme could be used for 
this purpose. The most highly developed method, 
however, uses Shoran. Special attachments have been 
developed for the Shoran airborne equipment to 
assist in the following two tasks: 

1. Keeping the aircraft on course. For maximum 
economy it is necessary to have the aircraft fly a 
series of courses such that the aerial photographs 
completely cover the desired area, but do not overlap 
excessively. In Shoran mapping, a series of parallel 


straight lines are flown. In trials, it was found readily 
possible to keep the aircraft within 500 feet of a 
course, so that complete coverage was assured (from 
15,000 feet) with 2% per cent overlap. 

2. Locating accurately one point on each photo- 
graph. If the aircraft is in straight and level flight and 
the axis of the camera is vertical, then the axial point 
on the photograph is directly below the aircraft. The 
aircraft position is known by recording the two bea- 
con- to-airc raft ranges at the instant of exposure. If 
the aircraft is not in level flight, but the angles of tip 
and tilt are known, the point on the photograph 
which is directly below the aircraft can be determined. 

The Shoran Photo-Reconnaissance Recorder unit can 
be installed in the airborne equipment in place of the 
usual bombing computer. It contains two variable 



DEVELOPMENTAL TRENDS IN BEACON BOMBING SYSTEMS 


127 


speed motors which can be adjusted by the Shoran 
operator to keep the rate and drift pulses continu- 
ously in alignment, that is, to measure continuously 
the beacon-aircraft ranges. A recording camera is in- 
cluded, which takes an exposure each time the aerial 
camera shutter is opened, and records the readings 
at that instant of the following instruments: the two 
Shoran distance (range) counters, the barometric 
altimeter, an aerial clock with a sweep second hand, 
a flux-gate compass repeater, a tip and tilt indicator, 
a data card, and an exposure counter. 

Mapping procedure is to fly to the area to be 
mapped, and by Shoran navigation to turn onto the 
first of the straight lines on which strip photos are to 
be taken. The Shoran operator keeps the pulses 
aligned and gives verbal instructions to the pilot to 
maintain the desired course. The taking of photo- 
graphs, and the recording of the data necessary to 
locate each photograph, is automatic. 

Tests by the Radar Laboratory, Wright Field, indi- 
cated that the geographical position of a point on an 
aerial photograph could be determined within 100 
ft, using this method. Points so determined are, of 
course, referred to the Shoran ground stations, and 
hence are known in absolute position to no greater 
accuracy than that with which the ground stations 
are located. Relative to other points determined by 
use of the same pair of ground stations, the accuracy 
quoted is good if the separation between the ground 
stations is precisely known. This separation can be 
determined, to within less than 50 ft, by flying a 
Shoran-equipped aircraft between the stations. 

Developmental work has also been done on auto- 
matic tracking devices, so the pulses do not have to 
be kept in alignment by manual adjustment, and on 
automatic computers which present the pilot with a 
PDI indication for flying straight line courses. 

10.7 DEVELOPMENTAL TRENDS IN 
BEACON BOMBING SYSTEMS 

10.7.1 General Deficiencies of 
Beacon Bombing 

Several important limitations exist to varying de- 
grees in all the beacon bombing systems described in 
this chapter. These are described in the following 
paragraphs. 

Range 

With all the radar-beacon schemes and with Gee 
the aircraft cannot bomb beyond the radar horizon. 


This limitation is so serious that in World War II 
the bulk of the blind bombing was done with radar 
bombing systems, although the accuracy of these 
bombsights was generally much less than that of 
beacon bombing schemes. It was unfortunately true 
that most of the important strategic targets were be- 
yond the radar horizon of friendly territory. 

Simplicity of Operation 

Excellent results have been obtained with most of 
these equipments by expert crews under practice 
bombing conditions. Operational errors were usually 
from three to ten times larger. An equipment which 
requires extended and concentrated attention by 
highly skilled operators is limited in combat effective- 
ness. It is therefore desirable to make the equipment 
reliable and fully automatic, or at least to reduce the 
duties of the air crew to as few and as simple oper- 
ations as possible. 

Computer Design 

Several of the beacon bombing schemes mentioned 
were limited in accuracy because the computer did 
not solve the bombing problem completely. In some 
cases, part of the computing problem had to be 
worked out by the man operating the equipment, 
thus increasing his duties and enlarging the chance 
of error. The most direct course to the target and the 
easiest to fly is a straight line, yet most computers 
permitted only an arc of a circle or a branch of an 
hyperbola. In no case did the computer output actu- 
ally fly the aircraft, and in only a few cases was the 
pilot given his angular deviation (heading error) as 
well as his displacement error (distance to right or 
left) from the correct course. Only tentative schemes 
have been proposed for computers allowing approach 
to a target from any arbitrary direction. 

Accuracy of range measurements, although far 
from perfect in any of these systems, is not listed here 
as a serious limitation, because the contribution of 
range error to bombing inaccuracy is small compared 
with the errors caused by lack of skill of the operators. 
With the best of present technique, range errors are 
roughly 15 yd. 

10.7.2 Methods of Increasing 

Range for Beacon Systems 

Two methods exist for increasing the range at 
which an aircraft, at a specified altitude, can work 
with ground equipment. The first of these is to ele- 
vate the ground equipment: the operating range is 


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128 


BEACON BOMBING 


proportional to (hi)'* + (W* where hi is the height 
of the aircraft and h 2 is the height of the cooperating 
equipment. The second is to relay pulses from ground 
equipment to aircraft and back by means of airborne 
repeater stations. 

Airborne Beacons 

A simple example of the first method was proposed 
to extend the range of the Troop Carrier version of 
Micro-H Mark II. Here the aircraft flies at 500 ft 
altitude. Lightweight beacons were to have been 
suspended from barrage balloons. If the two barrage 
balloons were moored so that the beacon elevations 
became 1,000 ft, the range of the system would be 
increased by a factor of 2.4, that is, from 30 to 70 
miles. 

A more elegant proposal is to make airborne the 
two beacons in an H system. A means must be used 
then to locate the beacons in space, and the beacon 
reply must be coded to include the beacon position. 
In one case, it was proposed to modify Micro-H 
Mark III by making the two AN/CPN-6 beacons 
airborne. The beacon equipped aircraft was also to 
carry a precision radar and a GPI computer (see 
Chapter 9). The aircraft was to fly about a clearly 
defined radar reference point of known position. The 
GPI was to resolve the aircraft displacement from 
this point into rectangular coordinates. The y axis 
of this coordinate system is the line joining the 
reference point and the bombing target. To a first 
approximation (see Figure 18) the range from the 
bombing aircraft to the reference point is the range 
from the bombing aircraft to the beacon plus the y 
coordinate of the beacon. If the reply of the beacon 
is delayed by an amount which is proportional to y 
plus a constant (the constant being added to elimi- 
nate the possibility of negative delays), the bombing 
aircraft sees the beacon at an apparently fixed point. 
The bombing aircraft can therefore carry the stand- 
ard H bombing airborne equipment and follow nor- 
mal H bombing procedures. 

If all three aircraft fly at an altitude of 30,000 ft 
the range of this system is 500 miles from the refer- 
ence points. This increases by a factor of four the 
area that can be attacked if the reference points are 
in friendly territory. (In this event, the reference 
points might be marked by ground beacons.) How- 
ever, it is also possible to choose clearly defined radar 
reference points in enemy territory, and then the 
area of attack is unlimited. The choice of suitable 
reference points is relatively easy, because any two 


points, each within 500 miles of the target, can be 
used, provided these points subtend an angle of at 

/ 



BEACON NO. 2 

Figure 18. H system with airborne beacons. 

least 30 and less than 150 degrees as seen from the 
target. It should be pointed out that the beacon 
equipped aircraft can fly any course it chooses, pro- 
vided it remains within approximately 20 miles of its 
reference point. 

The outstanding disadvantage of this scheme is that 
the coordinate systems have to be chosen for a par- 
ticular target. Therefore only one target area can be 
attacked at a time. This restriction can be removed 
by the following scheme: let the coordinate axes be 
oriented at will, say, let the y - axis at each reference 
point lie in a north-south direction. Both the x and y 
outputs of the GPI are fed to the beacon. The beacon 
replies with three pulses, the first of which is unde- 
layed, the second of which is delayed by an amount 
proportional to the ^-coordinate of the beacon posi- 
tion, and the third of which is delayed by an amount 
proportional to the ^/-coordinate of the beacon posi- 
tion. The beacon reply then contains all the informa- 
tion required to locate the beacon position. Decoding 
equipment is carried in the bombing aircraft, and this 


DEVELOPMENTAL TRENDS IN BEACON BOMBING SYSTEMS 


129 


information, when decoded, is supplied to a com- 
puter which computes the position of the reference 
point and the range from the bombing aircraft to the 
reference point. All the information needed to bomb 
an arbitrary target is therefore available in the air- 
craft. 

Repeater Schemes 

A method for extending the range of Oboe Mark I, 
which saw a few operational trials, depends on air- 
borne repeater stations. The Oboe ground station and 
Oboe bombing aircraft are unchanged (except in 
radio frequency) but a repeater aircraft flies along 
the line joining the ground station and the target. 
This aircraft carries two transmitters and two re- 
ceivers. It receives and retransmits pulses from 
ground station to the bombing aircraft and vice 
versa. Since the repeater aircraft can operate to the 
radar horizon of the ground station, there is an in- 
crease of a factor of three in range using this system. 
Frequencies are so chosen that when a pulse from 
the ground station is received it is repeated to the 
bomber receiver only, and conversely. 

With Oboe, difficulty was experienced because, in 
prototype equipment, a given receiver-transmitter 
pair was only 70 per cent reliable at operating alti- 
tude. Since the repeater system used three such pairs, 
the system was only 35 per cent reliable. The problem 
of getting normal Oboe in large-scale operation was 
very pressing at the time, so the repeater Oboe pro- 
gram never got beyond the stage of trial operations. 

Oboe errors introduced by the repeater aircraft are 
of two kinds. First, the repeater aircraft may deviate 
from its assigned altitude, or may be displaced later- 
ally from the course it is briefed to fly. For a repre- 
sentative situation, an error of 1,000 ft in altitude 
causes a bombing error of 40 yd, and a lateral dis- 
placement of 1 mile causes a bombing error of 7 yd. 
These errors are not serious. Second, as the repeater 
aircraft flies back and forth on its assigned track, the 
sum of the distances from the ground station to re- 
peater and from repeater to a point at some given 
altitude over target does not remain constant. (If we 
consider the three points as forming a vertical tri- 
angle, the base line remains constant, but the sum of 
the lengths of the other two sides varies as the apex is 
moved.) If the repeater aircraft is at 35,000 ft and 
the target is at 500 miles from the ground station, 
and if the repeater aircraft stays between 150 and 
250 miles from the ground station, the maximum 
error from this source is 90 yd. It is quite conceivable 


that this error could be substantially reduced by in- 
serting a variable delay in the repeater link which 
depends on the aircraft position on its assigned track. 

A similar repeater scheme was considered for use 
with Shoran. 

10.7.3 Methods of Simplifying 
Operation 

There are two ways in which the simplification of 
bombing procedure can be effected — either the 
bombing equipment can be made simple or it can be 
made automatic. Simple equipment with fewer con- 
trols provides less chance for maladjustment. More- 
over, the simple equipment is lighter and more reli- 
able in operation. It does have the disadvantages that 
less can be done with simple equipment and more 
depends on the skill of the operator. On the other 
hand, with automatic equipment, the operator’s skill 
plays a minor role while the resultant equipment com- 
plexity is not a disadvantage if reliable operation is 
insured. 

It is frequently overlooked that many “ simple” 
equipments obtained their simplicity by requiring the 
operator to perform duties a machine could do 
better. For example, straight Gee bombing requires 
no airborne computer and only simple equipment; 
but the operator is required to work out, with pencil 
and paper, an approximate solution to the bombing 
problem while the aircraft is approaching the target. 
Shoran carries an airborne computer and is therefore 
more complicated, but the operator needs only to 
keep pulses aligned for the bombing problem to be 
correctly solved and the bombs released at the right 
time. 

It is quite possible to conceive of an H system, for 
example, in which one simply sets into the airborne 
equipment the coordinates of the target and the bomb 
ballistics. The aircraft is flown to the general target 
area, the equipment is turned on, and then the 
duties of the air crew are over until bombs are away. 
Such an equipment would be complicated but could 
be made reliable and foolproof in operation provided 
the correct data were used. The additional weight 
would be justified every time more bombs were 
placed on the target, and would be justified many 
times over, each time a gross error was avoided. 

It is evident that the same line of approach is a 
fruitful one for use in guided missiles. If the sole duty 
of the air crew is to get the aircraft to the target area, 
a guided missile could be built with the same equip- 


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130 


BEACON BOMBING 


ment and so launched that it came into the control 
area. 

10.7.4 Computer Design 

Application of GPI 

The GPI solution of the bombing problem (see 
Chapter 9) seems a hopeful one for use with beacon 
systems. The accuracy of the GPI solution depends 
on the accuracy and frequency with which fixes are 
supplied to the GPI. With beacons, highly accurate 
fixes can be supplied as frequently as desired. Use of 
a computer of the GPI type would permit a straight 
line approach to the target in any direction and facili- 
tate the use of evasive action on the early part of the 
bomb run. 

The computer should, of course, determine the 
bomb release point and actually release the bombs. 
The aircraft may be flown by the computer, using 
computer control of the automatic pilot, or by a pilot. 
The requirement, in previous bombing schemes, for 
a PD I indication showing the pilot not only his dis- 
placement from track but his heading error, has al- 
ready been mentioned. In the GPI case, only one 


meter is required, since in the GPI solution it is 
necessary only that the aircraft be headed for the 
correct point and not that it approach on a specified 
track. The task of the pilot is therefore easier with 
the GPI computer. 

Universal Bombsight 

It is possible to design a system which would in- 
clude a visual bombsight, a high resolution radar 
bombsight, and a beacon bombing equipment. Many 
of the components of these equipments are common 
and could be combined; for example, a single bomb- 
ing computer might be used, with three different data 
inputs. The components would be so synchronized 
(as in Nosmo, Chapter 8) that when the beacon 
bombing method is being used, the visual bombsight 
is pointed at the target (whether the target is hidden 
or not) and so forth. An air crew would then have the 
opportunity to select the most useful of the three 
methods for any particular target, and could use the 
same equipment and many of the same techniques, 
whichever method was chosen. Further information 
on the beacons discussed in Chapter 10 can be found 
in the bibliography of Part II. 38 > 39 . «, 46, 60-64, 88, 93 , 

107-110 


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Chapter 11 

GROUND-CONTROLLED BOMBING 


11.1 PRINCIPLES OF GROUND- 

CONTROLLED BOMBING 

11 . 1.1 Introduction 

In tie European Theater of Operations during 
World War II, a system of ground-controlled bomb- 
ing was developed as a solution to the fighter-bomber 
control problem in the tactical air commands [TAC]. 
Initially utilized by IXth TAC, the use of such 
radar ground control spread throughout the various 
TAC’s in the European and Mediterranean Theaters of 
Operations [ETO and MTO] and was applied to both 
medium and fighter bombers. It has been termed 
“Close-Support,” “Close-Cooperation,” or “Close- 
Control” bombing. The fighter-bomber groups found 
the system particularly useful as no equipment had 
to be installed in the aircraft. Extension of the con- 
trol coverage required eventual introduction of air- 
borne beacons for longer-range low-altitude oper- 
ations. 

The close-cooperation bombing system provides 
accuracy of control from equipment that is flexible 
and mobile. Its tactical uses range from the simple 
provision of navigational direction to the target area 
to accurate positioning of aircraft for medium level 
bombing under conditions of complete overcast. The 
problem of location of the target, navigation to the 
target area, computing the ground-track-aiming-point 
[GTAP], steering the aircraft towards the GTAP, 
and computing the bomb release point are all solved 
on the ground. The pilot flies the aircraft according 
to instructions radioed from the ground-controller 
and releases the bombs when so directed. 

A ground-controlled bombing system contains five 
major components: 

1. An automatic tracking radar which accurately 
measures the slant range, azimuth angle, and eleva- 
tion angle of the target aircraft. Such a radar must 
have an excellent data transmission system which 
will provide the measured information in a form use- 
ful for controlling. 

2. An automatic plotting table upon which the 
position of the aircraft is continuously recorded. It 
is desirable to have the aircraft track superimposed 
on a map of the area so that the relationship of the 
aircraft to its assigned target is always apparent. 


3. A reliable communication link between the con- 
trolling radar and its target aircraft. An adequate 
communications network must also be provided for 
linking the controlling raddr with supporting and 
associated ground units of the command. 

4. A computer to determine accurately the GTAP 
and bomb release point by utilizing the output data 
of the radar. 

5. A beacon in the target aircraft for identifica- 
tion and for long-range low-altitude operation. Such 
a beacon utilizing multiple pulse interrogation and 
off-frequency coded response techniques increases the 
reliability and traffic capacity of the close-coopera- 
tion system (see Chapter 10). 


11.1.2 Theory 

In the ground-controlled bombing system, a com- 
plete solution of the bombing problem is simpler 
than in any other type of bombing device because 
V g , the ground velocity, can be determined directly. 
The geometry of the problem is shown in Figure 1. 
RQ represents the distance the aircraft would travel 
over the ground during the time of fall of the bomb 
if it continued on a straight course at constant speed 
after the bomb release point R is passed. RQ is the 
vector sum of RS, the distance the aircraft would 
travel in still air, and SQ, the distance it is carried by 
the moving air mass. (RS — Tie) is the horizontal 
distance the bomb would travel in still air and OB = 
SQ is the distance it is carried by the wind, with the 
result that the bomb impact point is B. The GTAP, 
P, possesses the geometric property that, with a 
given | V a |, | |, and W, all ground tracks must pass 

through P for the bombs to strike P; the ground- 
controller therefore vectors the aircraft so that it is 
following the proper ground track through P at the 
release instant. 

The position of P on the plotting surface can be 
calculated prior to the bombing run from meteoro- 
logical and briefed data. Referring to Figure 1, it will 
be seen that P is displaced from B by an amount 
proportional to the wind vector and to the wind- 
ward of the target, B. From the geometry, it is evi- 
dent that: 


BP 


W 

V a 


x I T, I 



131 


132 


GROUND-CONTROLLED BOMBING 



V ff = Ground speed 

Tff = Bomb trail (in yards) 


B 


Bomb impact point 


R = Release point 

Figure 1. Geometry of ground-controlled bombing as it appears to the ground-controller. 


Note that BP is usually insignificant (of the order 
of 10 to 50 yd) compared with bomb dispersion errors 
and other errors in the technique. Unless the wind, 
(W), is strong (for example, 50 to 100 mph perpen- 
dicular to the ground track), BP can be neglected 
and B taken as the GTAP. 

The position of R, the release point, is computed 
during the bombing run to the target by a measure- 
ment of W g or V g T f directly on the plotting surface and 
a measurement of the aircraft’s height with the radar. 
(Note: T f depends on the height of the aircraft above 
the target and is obtained for a given bomb type from 
the same tables that give the trail | | .) A line is 

drawn connecting P and the aircraft’s position. An 
arc of radius BQ = | T R | is inscribed on the line, 
and QR = V g T f is laid off from Q, thus establishing 
the bomb release point at R. When the aircraft 
reaches a point on the ground track corresponding 
to R on the plotting surface, the final release signal 
is given. 

Just what fraction of the above procedure is per- 
formed automatically for the ground-controller de- 
pends upon the type of bomb computer utilized 
with the plotting table. Computers are discussed in 
Section 11.2.3. 

11.2 EQUIPMENT AND TECHNIQUE 

The ground-control bombing system has been de- 


scribed in complete detail in various publications. 
24. 26 , 28, 29, 45, 68-70, 100-105 photographs of the SCR-584/M 

automatic tracking radar and the MC-627 automatic 
plotting table are shown in Figure 2 A and B. 

11.2.1 The Radar System 

The SCR-584/M radar is capable of operating 
with aircraft not equipped with beacons at ranges 
up to 70,000 to 100,000 yd depending on type of air- 
craft. The range accuracy under these conditions 
shows a probable error of 20 yd. A probable error in 
azimuth angle measurement of 1.6 mils may be ex- 
pected and the elevation angle probable error is also 
1.6 mils for elevation angles greater than 3 degrees 
over a flat surface. With the standard SCR-584 
radar, careless adjustment of the rather critical auto- 
matic tracking unit usually results in considerable 
deterioration of the angular accuracy. A new version 
of the SCR-584 known as the SCR-584X provides 
three times the angular accuracy, as it has a prob- 
able error of only 0.6 mil. Target aircraft equipped 
with beacons can be tracked and controlled out to 
the full 168,000-yd range of the plotter. A beacon 
has a response delay time that varies with the type 
of beacon and with the strength of the interrogation 
signal that triggers the beacon. This delay time ap- 
pears as a range error unless response delay-time 
compensation is provided in the ground radar. In 


v4 


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EQUIPMENT AND TECHNIQUE 


133 


the case of the AN/APN-19A beacon, the pulse- 
interrogation unit provided as an addition to the 
MC-627 modification kit has a variable delay-time 
adjustment built into the circuit. Furthermore, the 
beacon response is coded so that the SCR-584 oper- 
ator has positive identification of the aircraft he is 
tracking. 

The accuracy of the plotting equipment associ- 
ated with the MC-627 is sufficient to utilize fully the 
accuracy of the SCR-584/M and the data trans- 
mission system associated with it. 

11.2.2 The Plotting Table 

The MC-627 plotting table receives the radar range 
data from the SCR-584 position, converts it into the 
X and Y coordinates of ground range from the 
SCR-584 position, and plots the continuous airplane 
ground track on the plotting surface. The linear scale 
of the PT/61 plotter can be chosen to suit the opera- 
tion. For “rough” navigation, scales of 1:500,000 and 
1:250,000 are provided. A map of the chosen scale 
can be inserted beneath the tracing surface so that 
the position of the pen on the map indicates the cor- 
responding position on the ground that is directly 
underneath the aircraft. The realizable accuracy of 
this relative presentation depends almost entirely 
upon the accuracy of the maps. In general, the errors 
in the best available 1:500,000 or 1:250,000 scale 
maps are an order of magnitude greater than the 
overall error caused by the equipment. 

For expanded scale operations, the range of scales 
provided is from 1 : 20,000 to 1 : 112,000, and any scale 
between these limits may be used . An expanded scale 
map may be inserted and provision has been made for 
separate X and Y verniers to adjust for unequal map 
shrinkage, alteration in map materials, or printing 
errors. X-Y target coordinates are set into the table 
with calibrated parallax controls so that the target 
position lies at one of five arbitrarily selected points 
(the center of the surface or the center of any one of 
the four sides). The ground-controller vectors the air- 
craft so as to bring its ground track radially in to- 
ward the target point or the GTAP, and gives the 
release signal when the aircraft reaches the computed 
release point. 

n. 2.3 The Computer 


(see also Chapter 8). The bomb release point is de- 
termined essentially by distance measurements on 
the plotting table, and the accuracy of the determi- 
nation depends upon the constancy of the aircraft’s 
velocity and similar factors throughout the bombing 
run. This method has, among others, the disadvan- 
tage that if the radar signals from the aircraft fade 
just prior to bomb release, a release signal cannot be 
given and the run must be repeated. 

Synchronous Type 

As the name implies, the synchronous type of com- 
puter involves setting up in a machine a position and 
a rate which correspond to the position and rate of 
the pen as it moves toward the target. The bomb- 
trail, time of fall, and target coordinates (or func- 
tions thereof) are also set into the computer (e.g., the 
Norden bombsight rate-end), which then calculates 
the bomb release point continuously. The function of 
the operator is to keep the indicator on the computer 
synchronized with the position of the plotting pen 
as it moves along the table, varying both the rate 
(V g ) and the position knobs as needed. The release 
instant is indicated to the controller by the coin- 
cidence of two indicators on the computer. The ad- 
vantages of the synchronous over the impact-predic- 
tion type of computer are that it has a shorter time- 
constant, (i.e., less time is required to reach a new 
solution after a change in one of the variables), and 
it does not fail because of momentary fluctuations in 
position resulting from the loss of signals by the 
radar. It is also a reassuring device for the controller 
to operate, because the solution does not depend upon 
instantaneous measurements which are invariably 
poor when made under tension; instead, it depends 
on adjustments which can be checked and refined 
from time to time throughout the bombing run. 
Synchronous computers are much more complex 
than impact predicting computers. 

In the European and Mediterranean Theaters of 
Operations, Norden sight techniques of a semi- 
synchronous nature were developed and utilized to 
fill an urgent need until a fully automatic computer 
could be produced. They were not continuously ad- 
justable and consequently were not much superior 
to impact-prediction methods. 

Automatic Computer (Synchronous) 


Impact-Prediction Type 

The procedure for obtaining the release point as 
outlined in Section 11.2.2 is called impact prediction 


An automatic computer was developed by the Bell 
Telephone Laboratories and the Radiation Labora- 
tory for use with the MC-627 table. It provides com- 



134 


GROUND-CONTROLLED BOMBING 



Figure 2 A. Ghost view of SCR/584. 



EQUIPMENT AND TECHNIQUE 


135 



Figure 2B. Overall view of RC/294 plotting board. 



136 


GROUND-CONTROLLED BOMBING 


pletely automatic computation when the bomb bal- 
listics, true airspeed, altitude, wind velocity and 
direction, and target coordinates are set into it by 
means of knobs. The time constant of this computer 
is only 7 sec. Once adjusted for the problem, the com- 
puter then automatically provides the controller with 
information as to the angular correction required to 
correct the ground track of the plane. Also presented 
are the bearing angle to the GTAP, the ground speed 
toward the target and an indication of the time re- 
maining before release when the airplane is within 
100 sec of reaching the release point. The informa- 
tion furnished by the computer may be used by the 
controller in his voice control procedure or it may be 
sent out over the radio by an automatic steering angle 
correction circuit designed to operate a coded “on 
beam” indicator. This indicator is similar to that 
used with the airways A-N beams, but in addition it 
is provided with a sensitivity adjustment operated 
by the controller which effectively narrows the beam 
for closer control as the target aircraft nears the re- 
lease point. During the last 10 sec before release a series 
of warning pips is provided, ending in a 3-sec pip, the 
end of which constitutes the release signal. 

n.2.4 Vectoring 

Voice- vectoring is divisible into two types, the 
first of which involves giving small angular correc- 
tions or new headings in order to bring the aircraft's 
projected ground track over the GTAP, while the 
second provides angular corrections or headings plus 
check-turns (S turns) so as to bring the aircraft down 
a specified line toward the target. The latter pro- 
cedure is a standard ground controlled interceptor 
[GCI] technique and is less desirable because it is 
much more difficult for the pilot to accomplish with 
accuracy than the former. It also has the disad- 
vantage that in making the check-turn, so as to pro- 
duce a lateral displacement of the course by a few 
hundred feet, the pilot may turn as much as 20 to 30 
degrees away from the line through the GTAP and, 
if he should happen to release the bombs under these 
conditions, they will miss the target by several thou- 
sand feet even though the aircraft is right on the pre- 
scribed line. In bombing, heading errors are likely to 
be more important than position errors at the release 
point. 

11.2.5 Communications and Their 
Applications 

The transmission of intelligence is the key to the 


success of the entire system. The importance of re- 
liable radio communication between the aircraft and 
radar is obvious. Good communication from the 
ground radar to certain other radars, airfields, and 
communication centers of the Tactical Air Command 
is also vital; for over these channels come daily, or 
oftener, orders for the missions, coordinates of the 
targets, identification codes of the aircraft, ground- 
to-air communication channel frequency assign- 
ments, meteorological data, data on bomb load and 
type, take-off and over-target schedules, position of 
the bomb line, flak areas and forbidden areas, and 
other necessary operational and administrative in- 
formation. In this connection it must be emphasized 
that communications play a vital part in the actual 
bombing missions, because the aircraft or flight is 
controlled up to the rendezvous point and after 
bomb release by long-range GCI or MEW radars. 
Thus, the aircraft remains under tight ground control 
as it is handed from radar to radar or station to 
station. 

Both land-line and f-m radio or microwave com- 
munications are necessary in order to handle all 
situations. When the front is static, land-lines are 
preferred to radio channels because they are some- 
what simpler to maintain and their security is better; 
the radio channels are then used for emergency 
standby. In advancing or retreating situations, it is 
difficult or even impossible to maintain land-lines, 
and radio channels or couriers become the means of 
communication. 

n. 2.6 Large-Scale Unified Control 
Systems 

In the previous section it was hinted that the SCR- 
584 system is only the ‘ 1 sight ' ’ to a tremendous 1 1 gun 1 1 
comprising all the radars, radios, land-lines, control 
centers, etc., of the Tactical Air Command. One of 
the most successful of such large-scale systems existed 
in the IX Tactical Air Command, 555 SAW Bat- 
talion, and is represented in part in Figure 3. 

The fully-engineered control radar of which the 
SCR-584/M is the prototype never actually saw 
action. This final conception of a close-cooperation 
unit is termed AN/MPQ-2 and is accompanied by 
adequate tables of organization and equipment that 
include all radar, beaconry, VHF, f-m radio-teletype 
and telephone equipment needed to do the complete 
job. Sufficient mobile equipment and spares should 
be included to make the unit self-sustaining and 
fully mobile. 




* 4-1 


EQUIPMENT AND TECHNIQUE 


137 


TELEPHONE TO 
FIGHTER WING 
AND GROUP 



CONTROL 

REPORTING 

GROUND TO AIR (VHF) 


Elements of the 555th signal aircraft warning battalion fit together in a pattern like this. The MEW site, heart of the 
control setup, is usually within 10 to 30 miles of the front lines. Tactical missions can be directed from there, the two 
forward director posts, and the three SCR-584 control centers. 

Figure 3. A typical fighter and fighter-bomber control system for a tactical air command. 






GROUND-CONTROLLED BOMBING 


138 


In such unified systems some of the long-wave, 
older type radars as well as some of the recent ones 
have had real use. Height finders (AMES 13) ac- 
companied the MEW, while direction finders (SCR- 
575) accompanied the SCR-584’s. 

In supplementing and further modernizing such a 
system, it would be desirable (1) to incorporate 
beacon equipment with the MEW function; (2) to 
supplant all the AMES- 15 radars with AN/CPS-6; 
(3) to tie together by relay radar in a main control 
room all TAC, MEW, and AN/CPS-6 information, 
and in a master control room (by relay-radar) similar 
information from all the other TAC’s along the 
Army front. In this way, airtight control over all 
aircraft could be rigidly maintained. This would re- 
duce greatly the possibility of lost aircraft strafing or 
bombing friendly troops or of A A defenses firing on 
friendly aircraft. 


For round-the-clock ground control bombing to be 
practical, the TAC radar equipment should also in- 
clude (1) ground control approach landing systems 
(AN/MPN-1) at every major airstrip; (2) airborne 
radar reconnaissance squadrons equipped with mov- 
ing vehicle detectors like Butterfly (AN/APS-26), 
Firefly (AN/APS-27), or AMTI (see Chapters 23 and 
24). In addition, all aircraft should be equipped with 
beacons (such as the AN/APN-19A) so that they 
could be easily located, tracked, or identified at great 
ranges. All ground radars should be equipped with 
moving target indicators (even though the friendly 
aircraft are beacon-equipped), so that enemy aircraft 
can be readily spotted and identified. The airborne 
radar reconnaissance squadrons and night-photo 
squadrons should be operated under SCR-584 control 
so that the targets they spot can be accurately lo- 
cated on the situation map. 


t 


RESTRICTED 


Chapter 12 

APPLICATION OF RADAR TO TOSS BOMBING 


In certain bombing and fire-control efforts it has 
been deemed desirable to use visual sighting but to 
supplement this with radar range information. The 
logic of such a view lies in the fact that, whereas 
visual sighting can be reasonably accurate under 
favorable conditions and can often not be equaled 
with even very complicated radar equipment, radar 
range data will, in general, be far more accurate than 
such data obtained by other means and can be made 
available by the use of relatively simple radar equip- 
ment. Consideration of the use of radar range in air- 
borne gunnery will be made in Part IV of this vol- 
ume, so we will restrict our attention here to bomb- 
ing problems. 1 

An obvious application of radar range to visual 
bombing is the use of radar altitude (range to the 
ground) in adjusting a visual sight, such as the 
Norden, for a level bombing run. This procedure, 
which has seen considerable use because of the avail- 
ability of SCR-718 altimeters, would appear to have 
considerable advantage over the use of a pressure 
altimeter in operations above level terrain, since its 
use would obviate knowledge of the barometric 
pressure at ground level and the necessity of con- 
verting indicated altitude to true altitude. For pos- 
sible “universal” bombsights, which permit a bomb- 
ing run during which the altitude is changing, it 
would appear to be particularly convenient to use the 
continuous, reliable altitude information that a radar 
device can provide when over level terrain. Such ob- 
vious applications of radar range to the bombing 
problem are reasonably straightforward and it is 
therefore intended to devote the remainder of this 
chapter to a review of a type of bombing called toss 
bombing, in which, as will be seen, there is the possi- 
bility of a more intimate and unique use of radar 
range. Before taking up this particular problem in 
detail however, it might be pertinent to mention 
in passing the so-called “Sniffer” equipment 
(AN/APG-4), which is an f-m radar operating in the 
73-cm band. This equipment, by exploiting the 
Doppler effect (see Section 23.2.1), is capable of 
measuring a suitable linear combination of range and 
range-rate to semi-isolated targets over water and so 
permits automatic bomb release in a very low alti- 
tude level bombing run with altitude supplied, if 
desired, by an AN/APN-1 altimeter. In this connec- 


AN/APG-22 and AN/APG-24 of recent design, 
which could be used to provide range and range rate 
information. 

12.1 ANALYTICAL DISCUSSION 

12.1.1 Description of Toss Bombing 

In the type of bombing attack which has been 
given the name toss bombing an airplane makes a 
straight approach towards the target, but pulls up 
before releasing the bomb. Release should occur 
when a velocity component has been attained away 
from the previous direction of approach adequate to 
compensate for the gravity drop which the bomb will 
experience in its subsequent motion to the target. 
This type of attack is illustrated in Figure 1 in 
which two diving approaches are shown. 

If one compares this type of attack with a conven- 
tional level bomb run in which release is made at 
about the same range from the target, it is apparent 
that, in some instances, the trajectory of the bomb 
will be noticeably flatter for the toss bombing case. 
From the point of view of permissible tolerance in 
ranging accuracy, such a flat trajectory would be an 
advantage in an operation against a target of con- 
siderable vertical extent or, with proximity fusing, 
against a target in the air. A second favorable feature 
which has been claimed for the toss bombing tech- 
nique is that at the time the bomb is released the pilot 
has already commenced a maneuver which will soon 
take him away from dangerous proximity to the 
target. Aside from these alleged advantages of toss 
bombing — advantages whose validity will depend 
upon the exact nature of the attack — ■ it is clearly a 
very simple technique which can be used to advan- 
tage by the pilot of a single-seat airplane. Attack 
bombers or pursuit aircraft would appear to be 
particularly suitable for a toss bombing attack and 
it might be desirable in operations against some 
types of targets to supplement such bombing with 
machine gun fire. Despite the advantages claimed for 
the flatness of the trajectory of the bomb, the pull-up 
and subsequent bomb release must be controlled 
quite accurately in toss bombing. Although sponsor- 
ship of a toss bombing program has fallen in large 
measure to Division 4 of the National Defense Re- 
search Committee [NDRC], the possibility that 
radar can be a real aid makes it appropriate that the 


139 


tion attention is also drawn to the f-m equipments 



140 


APPLICATION OF RADAR TO TOSS BOMBING 


PU = POINT AT WHICH PULL UP IS INITIATED 
R = RELEASE POINT 



theory of the method be reviewed and the instru- 
mentation discussed in the following sections. 

Tactically, toss bombing attacks might be dis- 
tinguished according to whether the target is an air- 
borne one, such as a bomber formation which it is 
desired to break up for possible subsequent fighter 
action, or whether it is a surface target. Alternatively, 
one might distinguish between a level approach and 
the more general diving attack. In practice, a level 
approach will be preferable against airborne targets, 
whereas an attack against a surface target will almost 
always be necessarily a diving approach. As will be 
indicated, a ground target becomes for the purposes 
of analysis a moving one when there is a wind, so 
there is, in principle, little distinction to be made 
between aerial and ground targets. 

When the target is a moving one, or when there is a 
wind, it is desirable for the pilot to “lead” the target 
so as to achieve a straight collision course prior to 
pull up. This applies to both azimuth and elevation, 
although under some circumstances compensating 
factors (see “Effects Resulting from Attacking on a 
Pursuit Course” in Section 12.1.4) may result in good 
accuracy being obtainable when a pursuit course of 
small curvature is used. In attacking an enemy 
bomber formation, however, it would appear to be 
highly desirable to avoid the necessity of estimating 
what would be a large lead and attempt a straight 
head-on attack. 


12.1.2 Elementary Analysis of 
Toss Bombing 

Level Approach 

In analyzing toss bombing problems an important 
concept is the time to the impending collision. This 
quantity, which is commonly designated T c , is, at 
every instant of the approach, equal to the range to the 
target divided by the rate of closure. In the case of 
a level attack the problem essentially is to give the 
bomb an upward velocity (measurable electrically or 
mechanically as the time integral of the upward ac- 
celeration) which will cause the bomb to return to its 
original altitude in the time T c . If the forward mo- 
tion and displacement from the original flight path 
during the pull-up are ignored for the moment, such 
a consideration would lead one to the conclusion that 
in a pull-up begun when the time to the collision has 
a value T c , release of the bomb should occur when 



Here a is the upward acceleration ; g, the acceleration 
of a freely falling body; and T p is the time consumed 
in the pull-up before bomb release. 

As the attacking aircraft w T ill progress forward dur- 
ing the pull-up and also gain some altitude, the pre- 
ceding equation must be modified to the following 
form: 


RESTRICTED 


ANALYTICAL DISCUSSION 


141 


= + fa + l(X+l ~ l)]*, 


( 2 ) 


where T c , as before, represents the time to collision 
at the start of the pull-up. In this expression the 
second and third terms of the integrand provide the 
corrections, respectively, for the two effects just 
mentioned. (The second term, of course, would not 
be required if T c represented the time-to-go at release 
rather than, as here, the value just prior to the pull- 
up. This term has merely the effect of increasing the 
right hand side of the equation by the quantity T p .) 
In deriving equation (2) the assumption is made 
that the component of velocity along the original 
flight path remains constant and that, for obtaining 
the final term of the integrand, the acceleration may 
be taken as constant during the pull-up. Such as- 
sumptions may require further consideration later, 
but the relation expressed by equation (2) has been 
frequently referred to since February 1943. (For an 
early analysis of toss bombing, reference is made to a 
series of four reports by the Ordnance Dept., U.S. 
Army. 89-92 An extensive series of papers on toss bomb- 
ing has been issued by the Lukas-Harold Naval Ord- 
nance Plant. 106 ) 

Equation (2) illustrates the important role which 
the quantity T c plays in toss bombing and indicates 
the essentials of the problem to be solved. Basically 
what is required is, first, a means for obtaining T c 
and, second, a device for tripping the bomb release 
circuits when the integral has attained the proper 
value. Means by which these things can be done will 
be discussed in a later section, but it may be well at 
the moment to continue with the analytical aspects 
of the problem. 

It is convenient to introduce a quantity K to serve 
as a measure, in terms of g, of the spatial acceleration 
plus the gravity component; that is, for the level 
approach considered at present, 



Figure 2. Plot of the integrand of equation (4b For 
comparison the dashed line illustrates the proposed 
linear approximation (2.035) (A-0.322). 

and 4, we can replace equation (4) by the following 
approximate condition for release: 

T c = J'J“ 2.035 (K - 0.322 )dt, for 1.5 T K <C 4. (5) 
Diving Attacks 

In analyzing a diving attack we find that the air- 
speed of the airplane in its dive is involved ex- 
plicitly in addition to affecting the value of T c . With 
assumptions similar to those for a level approach we 
find that the toss bombing equation must be modi- 
fied to the following form for a diving attack making 
an angle a with the horizontal. 19 

T c = T p X \_K + VK(K - 1)] X 


M«,K,T C /V)’ (6a) 


where 


a + q a 

K =—^ = - + 1. 


Equation (2) can be rewritten in terms of K : 

Tc =f 0 Tp [ K + VK(K - 


(3) 

(4) 


^ = K + VK(K - 1) , ^ cos a 

K + Vk(K — cos a) 13 

L Vl + 2/3 - 1] , (6b) 

and 


It is interesting to note that, as shown in ref. 92, 
the integrand of equation (4) may be approximated 
quite accurately by a linear function of K if this will 
facilitate the instrumentation. A plot of this inte- 
grand and of the proposed linear approximation is 
given in Figure 2. Thus, for values of K between 1.5 


„ T c (K - cos a) g sin a 

0 = 77 ^ 

Note: K is defined as the total number of g’s — acceleration 
and component of gravity — perpendicular to the original line 
of flight, so that ( K — cos a) g is the spatial acceleration of 
the airplane. 


142 


APPLICATION OF RADAR TO TOSS BOMBING 


It is seen that the function ^ represents a correc- 
tion factor for equation (4) and takes on values 
running from 1.00 for a = 0 to 0 for a = 90 degrees. 
\p is primarily a function of a with only small de- 
pendence on T c /V and virtually no dependence on 
K. A plot showing the dependence of ^ on a for vari- 
ous values of T c /V (and for K = 4) is given in 
Figure 3. In practice, the dependence of ^ on K can 
be neglected and the effect of variations of T J V can 
be adequately compensated if T c is measured by the 
charging of a capacitor and the nonlinearity of the 
charging curve is exploited. 19 

12.1.3 Critique of the Elementary 
Analysis 

The Effect of Pull-up Angle 

Before considering mechanisms which can be em- 
ployed to measure T c and the other pertinent quanti- 
ties and to give the signal for bomb release, it may be 
well to review briefly the possible errors introduced 
by some of the assumptions made in deriving the 
general toss bombing equation (6a) and similar rela- 
tions. One assumption which can lead to appreciable 
error is that the pull-up acceleration, which is initially 
perpendicular to the collision course, remains con- 


stant in direction. This, of course, implies that the 
velocity component along the initial direction of 
approach remains constant. Actually a more realistic 
assumption would take the airspeed constant and the 
acceleration directed perpendicular to the instan- 
taneous direction of flight — in practice, the total ac- 
celeration (including the component of g) measured 
along such a direction would be the quantity most 
readily measurable by equipment mounted in the 
airplane. 

The result of assuming that the acceleration is 
normal to the initial direction of flight will be that the 
bombs actually released will fall short of the target, 
particularly if the pull-up is begun at long range. A 
quantitative study of the amount of horizontal range 
error introduced has been made at the National 
Bureau of Standards. 5 - 6 Approximate formulas are 
derived for the error and curves given to show the 
conditions under which a 100-ft range error will be 
obtained. Table 1 illustrates the information given 
by these curves. In general, the variation of this slant 
range is approximately as V s ' 2 when a is large (com- 
pared to the circular pull-up angle) and as V 4 3 when 
a is zero or small. Similarly, if we are interested in 
the values of these ranges for horizontal errors other 
than 100 ft, we may use the approximate relation 3 



1 igure 3. Plot showing the dependence of \J/ on a for various values of T c /V. (K = 4). 



ANALYTICAL DISCUSSION 


143 


that these errors vary approximately as the fourth 
power of the range when a is large and as the cube 
when a is close to or equal to zero. 


Table 1 . Slant range (yd) for which assumption of 
usual toss bombing equation will result in 100-ft 
horizontal error ( K = 3). 


Airspeed 



Dive angle 



(knots) 

0 

10 

(degrees) 
20 30 

40 

50 

60 

250 

1,160 

1,340 

1,420 1,570 

1,780 

2,100 

2,570 

300 

1,480 

1,730 

1,860 2,050 

2,340 

2,770 

3,380 

350 

1,830 

2,150 

2,330 2,570 

2,950 

3,300 

4,260 

400 

2,180 

2,580 

2,830 3,140 

3,600 

4,270 

5,200 

450 

2,550 

3,050 

3,370 3,740 

4,300 

5,100 

6,200 


It may be advisable to point out that the source of 
error just discussed does not appear to be a serious 
one. Frequently, release will be made at ranges con- 
siderably less than the values appearing in Table 1 
and, because of the third or fourth power law, the 
horizontal error resulting will, accordingly, be 
markedly less than the 100 ft taken for the purposes 
of illustration. Furthermore, if this error is regarded 
as arising from an incorrect value of T p , one can 
calculate 3,7 the percentage error in the value of T v 
resulting from assuming a constant direction for 
the acceleration in deriving the expression for the 
correction factors of equation (6a). This suggests the 
desirability of modifying the form of the function 
\p given by equation (6b) and illustrated in Figure 3. 
(yp as used here is sometimes designated \J/' in the 
paper of London. 7 ) Such a modified \J/, valid for pull- 
up angles less than 20 degrees, has been proposed 3>7 
and is illustrated in Figure 4. It is apparent that these 
new \f/ curves intersect at 17 degrees, instead of at 
0 degrees, and that they exhibit less variation with 
T C /V than did the unmodified curves. 

The Effect of Variable Acceleration during 
the Pull-up 

A second possible source of error in the solution 
of the toss bombing problem is the assumption of a 
constant acceleration during the pull-up. The effect of 
a nonconstant acceleration has been considered in the 
report by London 7 and, for a horizontal approach, in 
an earlier paper by McLean. 2 Types of acceleration 
curves which have been experienced 3)7 are (1) accel- 
eration increasing linearly with time, (2) acceleration 
increasing linearly until a definite value is reached, 


after which it remains constant, (3) acceleration in- 
creasing linearly, first at one rate and then at an- 
other, and (4) a linear increase followed by an actual 
decrease of the acceleration after a definite value has 
been attained. For the general case, the function 
K + V K(K — 1) which appears as an integrand in 
equation (4) (and in the analogous formula for non- 
horizontal approaches) should be replaced 2 by the 
function 

f{K) =K + 

avi' (A' - m 

- my + 2 fidtf 0 \K - m 

Using this function, the general toss bombing equa- 
tion will be [see equation (6a)]: 

T c = 7 ( Tp f(K)dt. ( 8 ) 

\pj 0 

If K is constant, the expression for f{K) reduces to 
K + VK(K - 1). If, instead, K varies linearly with 
time, the correct function is 


« - + + <»> 

The expression given in equation (9) has values 
commonly a few per cent less than the more common 
function K + VK(K — 1) and, as a result, the val- 
ues of T p derived by the use of the simple function as 
integrand would be expected to be a few per cent too 
small. Actually integrators which have been con- 
structed use in effect a function. 

f(K) = for K ^ 1.3 (10) 

f(K) = K + Vf(K-l) for K > 1.3, 

so a linearly increasing acceleration, for which equa- 
tion (9) should be used, is, in fact, quite accurately 
integrated by such integrators (see, for example, 
Figure 9 in reference 7). Similarly such integrators 
introduce only negligible errors of the order of 1 per 
cent when the other types of acceleration functions 
are involved. 7 It should certainly suffice to consider 
in this connection only values of K no greater than 
6 and probably values of dK/dt no greater than 6 
per sec. 


RESTRICTED 


144 


APPLICATION OF RADAR TO TOSS BOMBING 



Figure 4. Plot showing the modified function of a for various values of T c /V. The modification is intended to com- 
pensate Tp for the effect of pull-up. (K = 3). 


12.1.4 Examination of Additional 
Possible Sources of Error in 
Toss Bombing 

Effect of a Sight not Aligned with the Line 
of Flight 

The effect of making an approach in which the 
flight path before pull-up is not a collision course 
with the target has been investigated . 4 - 8 The results 
of this situation, which might arise through the use 
of a gunsight not exactly aligned with the line of 
flight, may be expressed in terms of the necessary 
compensation which could be introduced into the 
timing circuits. The percentage change required in 
the integrator ratio is roughly the same as the per- 
centage which the sight offset is of the pull-up 
angle , 4 which, in turn, depends on the range at which 
the pull-up is initiated. More accurate relationships 
are given in curves and formulas in the latter more 
comprehensive reference . 8 

Effects Resulting from Attacking on a Pursuit 
Course 

As a final point, it might be mentioned that in 
working papers of the Columbia University Applied 


Mathematics Group 56-59 an analysis has been made 
of the effect of an attacking aircraft making a pur- 
suit attack on a target in motion (more accurately, 
on a target in motion with respect to the air mass). In 
these papers the case considered specifically is that 
in which an optical sight can be depressed by the 
proper “lead” angle (with respect to the datum line 
of the aircraft) so that the bombs may be released at 
the instant in the pull-up when the target flashes 
through the sights. The use of such sights presents, 
of course, obvious physical and physiological diffi- 
culties. However, from the analysis it appears that 
there is an opportunity for some compensating fac- 
tors to enter. Thus, for a target motion directly away 
from (or toward) the attacking aircraft, the latter 
will experience an upward (or downward) accelera- 
tion as it proceeds along its pursuit course; as a result 
the dive angle, if measured by a pendulum device, 
will be in error. If this angle is taken in combination 
with the altitude to give a measure of the target 
range, the result will be the introduction of an excess 
lead not only in the proper sense to allow for the tar- 
get motion but, at large dive angles, of about the 
proper magnitude . 58 Likewise, when following a tar- 
get moving in azimuth, it is found that again there 


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INSTRUMENTATION 


145 


are some compensatory effects introduced. This is by 
virtue of the fact that the aircraft is obliged to bank 
in following the pursuit course and, in consequence, 
the lead (which is introduced in the plane of the 
aircraft’ s vertical) will place the sight not only above 
but also to one side of the moving target. 

12.2 INSTRUMENTATION 

From the previous discussion it is evident that, 
aside from the aiming of the aircraft (which is done 
visually), the solution of the toss bombing problem 
essentially requires the measurement of two distinct 
quantities. These quantities are (1) the time to tar- 
get, T c and (2) the time integral, from the start of 
the pull-up, of some function of the upward acceler- 
ation with proper dive angle compensation [see 
equation (8)]. In the simplest case these quantities 
might be regarded independently, in the sense that 
the value of T c at which the pull-up should be com- 
menced would be assigned in advance. Equipment 
would then be required to indicate to the pilot when 
the pull-up should be begun, and to release the bomb 
when the acceleration integral attained the correct 
value for the specified T c . A more flexible and con- 
venient arrangement, however, would be one in which 
the pull-up could be begun at the discretion of the 
pilot, and the value of T c for that moment compared 
with the acceleration integral. 

12.2.1 The Acceleration Integrator 

Acceleration integrators for obtaining the integral 
of a suitable function of the upward acceleration 
have been the subject of considerable developmental 
work by the National Bureau of Standards under 
NDRC auspices. Such devices could be constructed 
mechanically, for example, by using the extra weight 
which will result from an upward acceleration to im- 
part a torque and angular acceleration to a disk; 
when a suitable angular speed (which is the time 
integral of the angular acceleration) is built up, cen- 
trifugal force or other effects can be used to actuate 
the bomb release mechanism. Likewise, electrical 
methods — which may be more convenient — have 
been evolved in which the extra weight resulting from 
the acceleration is used to control the position of a 
slider on a variable resistor. The charging rate of an 
electric condenser can thus be made a suitable func- 
tion of the upward acceleration and, if the time 
constant of the charging circuit is made sufficiently 
long, the voltage to which the condenser is charged 


will be at every instant very close to an accurate 
measure of the required acceleration integral. Since 
the applicability of radar range information to the 
toss bombing problem is neither to the acceleration 
integrators nor to the devices for measuring the dive 
angle a we shall, in what follows, confine our at- 
tention to the measurement of T c and to those cases 
in which the value of T c is intimately incorporated 
into the acceleration equipment. 

12.2.2 Nonradar Methods of 
Determining T c 

Since the time-to-target, T c , depends not only on 
the range to the target but also on the relative rate of 
approach and is, in fact, equal to the ratio of these 
quantities, it is evident that something more than a 
mere estimate of range by stadiametric or other 
methods will be required. It is also evident that the 
rate of closure cannot be obtained satisfactorily from 
the airspeed of the attacker, since for a ground target, 
the effect of wind would then introduce an error, and 
for an airborne target, a precise estimate of target 
speed would be required. In the following paragraphs 
nonradar means of obviating these difficulties will be 
described and it will be seen that these methods 
possess the further advantage of not requiring a 
knowledge of the physical size of the target. It will 
be seen subsequently that these methods have their 
radar analogues, so they are presented for this 
reason as well as for completeness. 

Optical Methods 

In one optical method for determining T c use is 
made of a clock mechanism which can be started and 
reversed at will. If such a clock is started at the 
instant when a target subtends some given angle in 
an optical sight and is reversed when the subtended 
angle has doubled, it is clear that at the moment at 
which collision with the target would occur the clock 
would have returned to zero. The clock reading will, 
therefore, represent, at every instant after its re- 
versal, the time remaining before the impending col- 
lision. It has further been pointed out 91 that the ratio 
of the angles subtended at the times the clock is 
started and reversed may be something other than 1 
to 2 if a clock is provided which runs at different 
speeds forward and backward. Essentially what is 
being done, of course, is to obtain a measure of T c 
by timing an interval during which the distance from 
the target changes by a certain ratio ; following such 



146 


APPLICATION OF RADAR TO TOSS BOMBING 


a measurement an up-to-date value of this quantity 
is provided by continuously subtracting the time 
which elapses thereafter. 

By such a device we have an ingenious means of 
providing the acceleration integrator with the quan- 
tity T c which it needs to release the bombs at the 
proper instant. It is clear, furthermore, that this 
method does not require a knowledge of the dimen- 
sions of the target. In its practical application to air- 
borne targets in particular, this method has, how- 
ever, certain disadvantages connected with the fact 
that at long ranges stadiametric measurements on 
small targets may suffer from loss of accuracy, while 
possible measurements based on the entire width of 
a bomber formation may be invalidated by changes 
in the physical dimensions of the formation during 
the course of attack. 

Barometric Methods 

A method of determining T c which has some simi- 
larity to the use of the reversible clock is one involv- 
ing the use of a pressure altimeter in a straight line 
diving approach. In such an approach the relative 
change in the altitude differential between the at- 
tacking aircraft and the target is identical to the 
relative change in range to the target. Thus the time 
interval during which the altitude above the target 
changes by a definite fraction of its initial value will 
serve as a measure of the time T c remaining before 
the impending collision. Specifically, for example, if 
the time taken to dive from one altitude to another 
five-sixths as great is measured, the time T c remain- 
ing will be five times this time interval. More gen- 
erally, equation (8) could then be rewritten in terms 
of such a time interval in the following form 

(hjhi) - 1 ' <12 = t Jo }< ' K)dL (U) 

Here, of course, hi and h 2 are the altitudes above the 
target at the beginning and end of the time interval 
ti 2 being measured and the pull-up is presumed to 
start immediately thereafter. (Strictly what is re- 
quired is that the acceleration integrator be started 
functioning immediately at the end of the interval 
measured, in order that the value of T c so determined 
be used correctly.) 

The method just described has seen extensive use 
in flight tests, and has made use of a “ multiple point ” 
Kollsman aneroid altimeter in which contacts are 
spaced around the rim of the altimeter face at posi- 
tions such that the successive altitudes corresponding 


to these positions have the 5:6 ratio mentioned 
above. 19 A complete system employing such a device 
will be described in somewhat greater detail below 
but it might be appropriate first to point out here 
certain limitations which are inherent in this method 
aside from purely instrumental difficulties connected 
with data take-off problems and possible sluggishness 
in the barometric element. One fundamental limita- 
tion to the use of altitude variation for determina- 
tion of T c is its obvious uselessness in a horizontal 
approach, such as might best be employed against an 
airborne target. A difficulty which arises in a diving 
attack is the necessity for knowledge of the baro- 
metric altitude of the target. Conceivably, of course, 
this latter difficulty might be overcome by adjusting 
the barometric element while “ buzzing” a region at 
the same altitude as the target or by comparing the 
barometric reading with that obtained from a radar 
altimeter while flying over the area in question. These 
considerations are mentioned not to disparage the 
use of a barometric device for toss bombing, but 
merely to indicate some of the problems which may 
in some cases limit its applicability. Barometric lag 
is strongly affected by the nature of the installation 
and can be a real difficulty with some types of air- 
planes (as, for example, in the P-38 with the present 
type of static tube installation). 

Brief Description of a Toss Bombing System 
Employing a Barometric Element 

As indicated above, toss bombing tests have been 
made using a multiple point barometric altimeter for 
the determination, essentially, of the quantity T c . 
The other vital system elements are, of course, the 
acceleration integrator itself and the device which 
measures the dive angle, a, and provides the cor- 
rection factor t/'. To illustrate how such a system 19 
would work see Figure 5, in which charging circuits 
for two capacitors (Ci and C 2 ) are shown. The volt- 
ages which will be built up on these two capacitors 
will serve respectively as measures of \f/ • T c and of 
the acceleration integral J'f(K)dt. The equality be- 
tween these voltages will, in accordance with the 
basic equation (8), indicate the time at which the 
bomb should be dropped and can cause a thyratron 
(not shown) to actuate the bomb release mechanism. 

In continuing with the analysis of the circuit shown 
in Figure 5, we shall assume for simplicity that the 
two capacitors shown are equal, each having a capac- 
ity C. The potentiometer R a is intended to make 


available to the charging circuit RiCi a voltage which 


INSTRUMENTATION 


147 


is obtained by multiplying the supply voltage by the 
factor \J/, which represents the correction for dive 
angle. Accordingly the setting of the potentiometer 
R a is controlled by the equipment for measuring dive 
angle (for example, a free vertical-erecting gyro) in 
such a way -that the voltage obtained at the arm of 
the potentiometer is just \p • Vo. (Strictly, the value 
of \p, which is primarily a function of a, and shows 
negligible dependence on K, exhibits some variation 
with T c /V. How some allowances may be made for 
this fact will be indicated later.) 

To indicate how the quantity T c is introduced into 
the circuit , we now consider the action of the multiple 
point barometric element described previously. Use 
is made of this unit after the pilot has entered his 
dive, attained a reasonably constant airspeed, be- 
come lined up on the target, and has reached the alti- 
tude at which he wishes to begin the final stage of 
the attack. The pilot then closes a switch (“pickle 
switch”) which is at his disposal with the result that, 
at the next contact made by the barometric element, 
switch Si is closed automatically and the capacitor 
Ci begins charging. When the altitude has decreased 
to a point where a second contact is made by the al- 
timeter, switch Si is again opened and switch S 2 is 
closed; the charging circuit for C 2 is thereby con- 
nected to the voltage source and the pilot is given 
the cue to pull out of his dive. We are left mean- 
while with a charge stored on Ci, which, if linear 
charging can be assumed, 48b results in a potential 
given by: 

V c (12a) 

K iC 



Note. The possibility of some nonlinearity in the charging 
circuit can be exploited 19 to make some allowance for the 
slight variation of \p with TJV. If some value of V is taken as 
typical, an unusually large value of T c would result in the 
nonlinearity of the charging circuit becoming particularly im- 
portant and a somewhat reduced value of Vci being obtained. 
This effect is therefore in the same sense as would be obtained 
(for the usual values of «) by altering xp in accordance with its 
dependence on T c /V and evidently 19 satisfactory compen- 
sation can be obtained in this way. As a further remark it 
might be added that presumably the voltage divider repre- 
sented by the potentiometer R a is a device of sufficiently low 
impedance that variation in its setting will not result in a 
harmful variation of the total effective resistance in the 
circuit R 1 C 1 . 

The closing of S 2 starts the process of charging the 
capacitor C 2 through the resistor R 2 . This resistor is 



Figure 5. Schematic diagram to illustrate a possible 
toss bombing circuit. 


the heart of the acceleration integrator and its value 
will be dependent upon the amount of acceleration 
to which the aircraft is subjected. Again assuming 
linear charging, the potential built up on C 2 in a time 
t will be 


0 «/ 0 R 2 


(13) 


The value R 2 of R 2 should be arranged to vary with K 
so that 


R2 = 


R 

f(K)’ 


(14) 


where R is a constant and }{K) may be taken 
as K + y/ K(K — 1) or a suitable approximation 
thereto. We accordingly can express V Cv as given by 
equation (13), in the following form: 

sd£ m><u - <i5) 


As stated above, the bomb release will occur when t 
has attained such a value that V c 2 has become equal 
to Vci) in order that this release satisfy equation (8), 
a comparison of equations (12b) and (15) shows that 
we should put 

e , - eg - 1) <«) 

In the particular case cited previously as an example, 
hi/h 2 = 6/5, so the relation expressed by equation 
(16) would then become 

Hi = (17) 

5 


After the bomb has been automatically released an 
indication of this fact is, of course, given to the pilot 
and he is free to take whatever evasive action he may 
wish. In addition to his responsibility for uncaging 
the gyro, which is used to measure the dive angle, and 


148 


APPLICATION OF RADAR TO TOSS BOMBING 


taking care to prevent its tumbling thereafter, this 
type of bombing attack 98 requires of the pilot little 
more than a careful visual aim, some attention to 
the constancy of airspeed, and a reasonably abrupt 
pull-up in the plane of the aircraft’s vertical. 56_59, 98 
A further advantage is that the entire cycle of oper- 
ation is accomplished in a few seconds, so the pilot 
has the safety of a short bombing run. 98 If, with 
equipment based on the scheme described above, the 
pilot releases the pickle switch after having closed it 
at some point during the dive, the operation of the 
equipment is stopped and the apparatus is ready for 
operation at a lower altitude. It is evident that vari- 
ations and refinements of the circuit described are 
possible, but the above rudimentary material was 
presented in order to illustrate how the principles de- 
scribed earlier may be applied. One possible refine- 
ment, of course, would be a provision to compensate 
for the very slight deviation of the bomb trajectory 
from the theoretical path it would follow in the ab- 
sence of air resistance. Such a correction may, to a 
first approximation, be applied by multiplying T c by 
a factor slightly larger than unity which varies 
linearly with the range and in general is small enough 
that an average value of range can be used. 

12.2.3 The Use of Radar in Toss 
Bombing 

From the foregoing discussion of possible non radar 
means of instrumenting a toss bombing attack, it is 
immediately evident that radar range could be intro- 
duced into the technique in a quite straightforward 
manner and with possible advantages from the point 
of view of convenience or accuracy. Other more 
subtle and intimate means of introducing radar range 
have also been proposed, but in what follows, we shall 
consider first the radar methods analogous to the non- 
radar schemes already described and will begin with 
the use of radar altitude. 

The Use of Radar Altitude 

From the discussion of barometric methods in Sec- 
tion 12.2.2, it was noted that such methods may be 
handicapped by the lack of information concerning 
the barometric altitude of the target and conceivably 
could suffer from possible sluggishness in the equip- 
ment. It is natural to inquire, therefore, whether a 
radar measurement of the height above the terrain 
would not be more suitable. 

One method of obtaining and using such radar alti- 
tude information would employ an airborne range 


only [ARO] equipment such as the AN /APG-5 (see 
Section 20. 3), 22 although the use of microwave fre- 
quencies is by no means essential. (It might be de- 
sirable in using AN/ APG-5 equipment for this pur- 
pose to provide an antenna mounted in a different 
fashion from that customary in fire-control appli- 
cations. 22 ) The use of ARO could be quite analogous 
to the use of a barometric element, since the servo 
unit designed as an adjunct to the ARO automati- 
cally provides range as a shaft rotation, and a set of 
contacts could be provided to function in the same 
way as those on the pressure altimeter. The ARO 
equipment, primarily designed for ranging on other 
aircraft, normally provides the range to the nearest 
target in its fairly broad field-of-view and there 
should be no difficulty in obtaining the range to the 
closest point on the earth’s surface if the range 
covered by the servo unit were made adequately 
great. 

It will be immediately recognized, however, that an 
approach over rough terrain would make radar alti- 
tude information completely useless for the purpose 
considered here. In fact, even over water or relatively 
smooth terrain there will be some fluctuations in the 
output 22 which, from the standpoint of accuracy, 
would set a practical lower limit to the difference of 
the altitudes hi and h 2 between which measurements 
are made. Thus even under favorable conditions, the 
time required to determine T c with suitable accuracy 
might be about 3 sec and it could scarcely be claimed 
that the use of radar would permit a virtually in- 
stantaneous determination of this important quan- 
tity. Effects of this nature, which will be discussed in 
further detail in the following subsection, suggest a 
reason why there would be less benefit than one 
might expect in attempting to differentiate the alti- 
tude reading and provide T c essentially by furnishing 
instantaneously both the altitude, h, and its deriva- 
tive, dh/dt. In the summer of 1945, tests with ARO 
radar equipment replacing the barometric altimeter 
were being seriously considered by personnel of the 
Army Air Forces Proving Ground Command (Eglin 
Field). 

In both the barometric and the radar methods for 
the determination of T c by altitude measurements, 
arrangements might be contrived for commencing the 
measurement procedure the moment the pickle switch 
is pressed, without waiting until the next one of a 
discrete number of contacts is reached. With our at- 
tention now directed towards the application of radar 
to the toss bombing problem it would seem inappro- 


w 


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INSTRUMENTATION 


149 


priate, however, to discuss this particular point in 
connection with the use of radar altitude, since our 
interest in this particular method is rather consider- 
ably reduced by the difficulties which perforce arise 
in operations over rough terrain. It might further be 
pointed out that, just as with the barometric altim- 
eter, the use of radar altitude will not permit a de- 
termination of T c to be made in the case of a level 
approach, such as might be used against an airborne 
target. 

The Direct Use of Radar Range Measurements 
to the Target Itself 

In considering the use of radar range to the target 
itself as an aid in solving the toss bombing problem, 
ARO equipment again might appear suitable. Since 
the radar beam from the antenna of such equipment 
in its standard form is fairly broad (width to half- 
power points, 28 degrees), there would be little 
danger of losing the target in making a leading at- 
tack. It would be admittedly a difficult problem to 
use such equipment for direct ranging against a target 
on the ground, but attention is directed to the exist- 
ence of somewhat more complicated equipment like 
Vulture (AN/APG-13B) and Terry (AN/APG-21), 
designed to measure range to such targets for fire- 
control applications (see Sections 20.5 and 20.6). The 
adaptation of Terry equipment to this application 
would appear in fact to be a potential means of in- 
creasing its usefulness as a universal fighter radar. If 
direct range measurements are desired to an isolated 
target on the surface of the water, it should be pos- 
sible to modify the ARO equipment so that it will 
measure the target range instead of the altitude sig- 
nal, which is a signal necessarily at a closer range than 
the target itself. In an attack against an airborne 
target, however, the range to the target will some- 
times be greater than and in other cases less than the 
altitude and, in fact, may pass from the first of these 
conditions to the second during the interval in which 
T c could be measured. Under such circumstances, 
therefore, it would be desirable to shield the radar 
antenna in such a way that no altitude signal would 
be received; this should be possible in general, al- 
though a careful installation study would be required 
for each type of airplane involved. 

The range at which present ARO radar equipment, 
if operating at peak performance, can obtain a signal 
of adequate strength from even a single bomber-type 
aircraft should be sufficient (at least 3,000 yd), for 
toss bombing purposes. Adequate range performance 


certainly can be expected from surf ace- vessel targets. 
By methods analogous to those discussed in Section 
12.2.2, range data derived from the shaft of the ARO 
servo unit could then be readily used to measure the 
time interval during which the target range changes 
by a definite fractional amount and so serve, essen- 
tially, to provide a measure of T c . It would seem rea- 
sonable, however, to consider means for avoiding the 
use of the servo unit mentioned above. Since the ARO 
range unit contain^ automatic range-tracking circuits 
which provide a voltage varying in a precise linear 
manner with the range of the target, the servo unit 
does no more than convert this voltage into a shaft 
rotation. The elimination of the servo unit would, of 
course, necessitate the manufacture of an additional 
electronic unit to make use of the range voltage out- 
put from the ARO, but would have the advantage of 
avoiding possible sluggishness in a mechanical unit 
such as the servo. (The speed of response of the servo 
would perhaps only be important in the first stages 
of getting onto the target, but this aspect might be a 
real difficulty in a toss bombing attack, in which 
very little time is available. Actually, the production 
servos were capable of speeds greater than 500 yd per 
sec on the 2,000-yd scale.) By voltage-comparison 
circuits, 47a the range unit voltage can be used to con- 
trol circuits or mechanisms which will, as before, 
measure the time interval between any two desired 
ranges. The use of such methods was once considered 
for the adaptation of the longer wave AN/APS-16 
tail-warning equipment to toss bombing. 

The Use of Radar Range and Range-Rate in 
Toss Bombing 

As was pointed out in Section 12.1.2, the quantity 
T c is at every instant equal to the range to the target 
divided by the rate of closure. It would therefore ap- 
pear reasonable to inquire whether a measurement of 
range rate could not be obtained and applied to the 
solution of the toss bombing problem. The frequency 
shift which, by virtue of the doppler effect, is experi- 
enced whenever a target is in motion with respect to 
the radar could possibly be exploited to measure 
range-rates for toss bombing applications. In point of 
fact, in the design of the Sniffer equipment mentioned 
briefly in one of the introductory paragraphs of this 
chapter, the doppler effect has been used to provide 
range-rate information but essentially in a linear 
combination with range. The possible usefulness of 
AN/APG-22 or AN/APG-24 equipment, also re- 
ferred to earlier, should not be overlooked. In what 


150 


APPLICATION OF RADAR TO TOSS BOMBING 


follows, however, we shall consider exclusively the 
use of range rates obtained by the differentiation of 
range voltages, such as are obtainable conveniently 
with pulsed radar equipment like the ARO. It should 
perhaps be pointed out in advance that the deriva- 
tion of such range rates may not mean that a de- 
termination of T c can be made instantly , since the 
rates obtained may require a certain amount of 
smoothing which would in turn require that a certain 
time elapse before an accurate rate can be considered 
established. When this aspect of the problem is con- 
sidered it appears that, aside from possible conven- 
ience or elegance in the instrumentation, there may 
be little fundamental advantage to be obtained in 
practice from the explicit use of range rates in toss 
bombing instead of the “two point’ ’ methods con- 
sidered previously. 

Differentiation of a range voltage to obtain a range 
rate may be achieved in a very approximate manner 
by a measurement of the voltage drop across the re- 


sistor of a series resistance-capacity circuit although 
more involved circuits of suitable accuracy have been 
constructed for the purpose . 480 Once obtained, a volt- 
age representing the rate of closure may be employed 
in a variety of ways in the solution of the toss bomb- 
ing problem. In order to avoid the inconvenience of 
instrumenting a direct division 48d of range by range- 
rate to obtain T c , it is expedient to contrive methods 
for which this division will be unnecessary. 

A particularly straightforward use of voltages rep- 
resenting range and range-rate can be made if a par- 
ticular value of T c at pull-up is chosen in advance. In 
such a case the voltages representing these factors, 
with suitable proportionality constants, can be com- 
pared in a differential amplifier circuit, 47 * for example, 
and, when the range has decreased to a value equal 
to the assigned T c times the rate, a relay in the plate 
circuit will be actuated to start the acceleration inte- 
grator and give the cue to the pilot to begin the pull- 
up. The factor \f/ for dive-angle compensation could 



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RESULTS 


151 


presumably be introduced by use of a potentiometer 
in the range voltage channel of the circuit; further- 
more, the value of T c used can be readily selected by 
means of a potentiometer in the rate circuit if a suit- 
able adjustment of the acceleration integrator is made 
concurrently. A circuit of this type, without dive 
angle compensation, was constructed and used in con- 
junction with an early ARO equipment for toss 
bombing tests at the Army Air Forces Proving 
Ground in the spring and early summer of 1943. A 
block diagram of this system is shown in Figure 6 in 
combination with an early acceleration integrator. 
(The circuit of this “ calculator ” is given in Dwg. 
A 14995- A of the Radiation Laboratory, Massachu- 
setts Institute of Technology, and a block diagram of 
the entire system in Dwg. A14994-A. Some remarks 
on its performance will be made under Section 12.3.2.) 
Many variations of the detailed means by which 
range-rate can be applied to toss bombing will, no 
doubt, occur to our readers, but it might be of in- 
terest to present in the following paragraph one other 
method which has some unique features. 

An interesting type of toss bombing device which 
has been designed in laboratory form (again without 
dive-angle compensation) varies the quantity to be 
integrated by the acceleration integrator in direct 
proportion to the range-rate. A voltage comparison 
circuit then effects the release of the bomb when the 
integrator output becomes equal to the range voltage. 
The circuit of this calculator is shown in Figure 7. It 
is evident that such an arrangement permits the pilot 
to begin his pull-up at any time he may choose, pro- 
vided only that the radar is given sufficient time to 
establish accurately the range and (more particularly) 
range-rate voltages. It might be noted, however, that 
the usual toss bombing equation will require modifi- 
cation if up-to-date range data are used during the 
pull-up, since the relationships developed previously 
were based on the supposition that T c would be de- 
termined at a moment prior to the pull-up and the 
integrator would allow for the subsequent forward 
motion. A final practical point which should not be 
overlooked is that with such a device one must insure 
the continued reception of a radar signal from the 
target throughout the pull-up until the moment of 
bomb release; this may in some cases prove a re- 
quirement difficult to meet if it is also required that 
no altitude signal be received during level flight. 

12.3 RESULTS 

Toss bombing tests in which radar data were used 


are neither so numerous nor so recent as are non- 
radar tests of this technique. In the following sections 
no attempt will be made to digest the results of all 
toss bombing tests made to date, but a brief indica- 
tion will be given of the accuracies which can be ex- 
pected at the present time. It should be noted in 
passing that toss bombing Requires quite accurate 
timing on the part of the equipment — and, by the 
same token, on the part of test equipment for calibra- 
tion — since the total time consumed in measuring 
T c amounts usually only to a few seconds and the 
pull-up time before bomb release is typically little 
more than one second. 

12.3.1 Nonradar Toss Bombing 
Tests 

It is sometimes convenient, of course, to be able to 
test separately the various components of a bombing 
system and this can be done to some extent in toss 
bombing through the use of markers on the ground, 
stop watch or theodolite methods, and the like. The 
greater part of the testing, however, appears to have 
been done directly by actually observing drops made 
with practice bombs. 

One series of toss bombing tests carried out by the 
U. S. Navy involved drops from three SB2C and one 
F6F aircraft at slant ranges varying from 11,200 to 
4,100 ft. 96 In this series of runs, diving attacks were 
made against markers on the surface of the water and 
a multiple-point pressure altimeter served to meas- 
ure T c . The results quoted showed an average range 
error of 8634 ft attributable to inherent equipment 
error and pilot sighting. It was recommended that 
until a drift-computing sight should be developed, 
“the practice of diving with or against wind and/or 
target motion be employed to make deflection correc- 
tion easier, aiming off to compensate for drift.” 
The limitations of a radar altimeter were also indi- 
cated. 

It is interesting to note that, in an earlier series of 
over-water toss bombing tests made in connection 
with this project, 95 use was made of an AN/APN-1 
type (AYF) radar altimeter. Preliminary tests with 
this method of timing resulted in an average range 
error of 45 ft and led to the recommendation that 
“encouragement be given to development of AIBR 
(Acceleration Integrator Bomb Release) combined 
with the AN/APG-4 (Sniffer).” It is understood that 
further tests showed difficulties attributable to time 
lags and other inaccuracies of the AYF altimeter. 



AN-3I02-20-IP 
FROM A R 0 


152 


APPLICATION OF RADAR TO TOSS BOMBING 




Figure 7. Electric acceleration integrator and release computer. 



RESULTS 


153 


More recently an overall test of the AN/ASG-10 
(XN) equipment, consisting of acceleration inte- 
grator equipment based on designs of Division 4 of 
NDRC and a Kollsman pressure-type altimeter, was 
made in an F4U airplane. 97 Specifically the equip- 
ment comprised (1) a computer, Mk 20, Mod. 0; 
(2) an altimeter, Mk 1, Mod. 00; (3) a dive angle 
gyro, Mk 20, Mod. 0; (4) a control box; and (5) an 
indicator light. A stationary water target was used 
and attacks were run with a 40-degree dive angle, 
an indicated airspeed of 340 knots, and release at a 
slant range in the neighborhood of 7,500 ft or, in 
some runs, of 5,000 ft. The average radial error for 
the bomb drops made in the course of these tests was 
152 feet. The AN/ASG-10 equipment can also be 
used for rocket launching. 

The report 97 of the tests mentioned above points 
out that the “AN/ASG-10 equipment does not com- 
pensate for wind effects and the pilot must make the 
proper allowance in his sighting.’’ The report states 
further that “in these tests the pilots . . . calculated 
the point of aim from the best upper wind data avail- 
able and made their first runs using this point of aim. 
They then adjusted the point of aim for subsequent 
bombs to bring them on the target. Tests were not 
made on a moving target because no suitable target 
boat was available during the period of these tests.” 
It might be added that the wind speeds averaged 
about 25 mph and that runs were made up wind, 
down wind, and cross wind. As a second point, it was 
found that an adjustable sight is desirable, if it is to 
be usable for gunfire and for conventional dive bomb- 
ing as well, since the sight should be set about 30 mils 
above the boresight datum line when making a 40- 
degree dive at 350 knots. Finally, it might be noted 
that, for the installation used in these tests, the sus- 
pected lag of the pressure altimeter was “small and 
can be disregarded.” Additional tests are described in 
a report from Williams Field. 87 

12.3.2 Tests of Toss Bombing 

Methods Employing Radar 


a calculator unit, which employed range rate in the 
determination of T c and was constructed along the 
lines indicated in “The Use of Radar Range and 
Range-Rate in Toss Bombing,” Section 12.2.3 and 
Figure 6. Also, one of two Bureau of Standards ac- 
celeration integrators (without dive-angle compensa - 
tion) could be selected for us<e with this radar equip- 
ment. These units were mounted in a B-25D air- 
plane for the purpqse of the tests. 

Tactical Use Visualized 

The primary tactical use which was foreseen for the 
toss bombing technique at the time of the Eglin tests 
was for air-to-air work, in which a fighter plane (such 
as a P-38) or a fast attack bomber (such as an A-26) 
would make a toss bombing attack on a formation of 
hostile bombers in the hope of breaking up the for- 
mation and making it more susceptible to conven- 
tional fighter action. A direct hit on a member of the 
bomb formation was not considered necessary, since 
proximity fuses could be used. 

Features of Tests 

Despite the greater importance which was attached 
to air-to-air work at that time, many tests were 
made against ship silhouettes in order to test the 
equipment and the method more conveniently. It will 
be recalled that before the advent of Terry equip- 
ment the use of radar in toss bombing appeared to be 
most suitable for airborne targets, which are attacked 
without diving, or against targets on the surface of 
the water. The B-25 airplane was scarcely capable of 
as rapid rates of climb as would be desired in combat, 
but it served nicely for the purpose of the tests. Be- 
cause of the early form of the equipment and the 
experimental nature of the tests, an operator rode in 
the nose with the radar units and insured that the 
proper signals were received. In the runs against 
water targets, as well as in the air-to-air work, the 
attempt was made to approach at an altitude as 
close as possible to that of the target, since no dive 
angle compensation was then available. 

Some Difficulties Encountered 


General 

Various methods of employing radar ranging in the 
solution of the toss bombing problem have been men- 
tioned previously in this chapter but, in what fol- 
lows, we shall refer in particular to the series of tests 
run at the Army Air Forces Proving Ground (Eglin 
Field) during the spring and summer of 1943. In these 
tests use was made of an early ARO equipment plus 


It was found early in the tests that although the 
ARO equipment presumably gave quite accurate 
range voltages, rates obtained from these voltages by 
differentiation showed rather large and rapid fluctu- 
ation. Very roughly, the fluctuations experienced 
might be judged to have a period of the order of a 
second and some filtering was clearly essential in 
order that an accurate measure of T c be obtainable. 


* 


;tricted 


154 


APPLICATION OF RADAR TO TOSS BOMBING 


It was, on the other hand, of real importance to avoid 
introducing a filter with a slow transient response, for 
then an excessive time would be required to establish 
an accurate rate at the output of the filter. A reason- 
able compromise in filter design resulted in a three- 
stage rate filter which had considerable attenuation 
for frequencies of the order of 1 c but which would 
respond to a step function almost completely (within 
3 per cent) in 3 sec. (To reduce somewhat the time 
required to establish the correct rate voltage at the 
output of the filter it was suggested that the filter be 
charged to some intermediate value until a radar sig- 
nal is obtained from the target and range tracking be- 
gins. However, such obstacles as electrical leakage in 
certain relay contacts introduced difficulties in the 
exploitation of this idea in the tests being discussed 
and full use was not made of the suggestion.) With 
such a filter it then became necessary that radar data 
be obtained for at least 3 sec prior to the pull-up time 
in order that an accurate determination of this in- 
stant be obtainable. 

In the B-25 installation the altitude signal re- 
ceived by the radar was not found to be strong, al- 
though no great pains had been taken in mounting 
the antenna to avoid it. It was, however, necessary 
for the operator to guard against the occasional pos- 
sibility that the range tracking circuits would lock 
onto the altitude return instead of the signal from the 
target. In air-to-air tests a radio-controlled drone 
(PQ-8) was used as a target, sometimes flanked by 
one or two other small aircraft. Because of the low 
radar cross section of such targets — plus the fact 
that there was no AFC nor, at that time, exceptional 
stability in the. r-f circuits — it was desirable for the 
operator to check the tuning occasionally in the 
course of flight. 

With some improvement in these design features of 


the radar equipment and with the larger cross sec- 
tions presented by bomber type of aircraft, there is 
perhaps no essential difficulty with respect to maxi- 
mum range. If, then, the altitude signal were elimi- 
nated, it would appear entirely possible for the radar 
to feed data to the other equipment and to the pilot 
with no attention from an operator. In brief, the 
radar would search in range until picking up the 
target, at which time the so-called clamp voltage 
would cause automatically an on-target light to come 
on and range information would be made available to 
the calculator. Then, when the range decreased to 
such a value that it equaled the assigned T c times the 
range-rate, the integrator would start, as before, and 
the pull-up light give the cue to the pilot. 

Accuracy 

In tests against water targets with the equipment 
described above, an average range error of 30 ft is 
quoted. The tests were made with values of T c of 7 or, 
occasionally, 9 sec. In the work against airborne tar- 
gets the range errors were roughly three times as 
great and it was estimated that the average vertical 
error was approximately 30 ft. 

Such accuracies were felt to be definitely encour- 
aging, particularly in view of the rudimentary nature 
of the equipment used. It was recognized, however, 
that many engineering and production problems 
would require solution before combat use could be 
made of equipment of this nature. The indiscreetness 
of introducing such a technique at a time when the 
Allied bombing effort was on the increase led to dis- 
couragement of a future program for air-to-air toss 
bombing. For air-to-air work particularly, radar 
range data can certainly be of value and methods 
such as those discussed in Section 12.2.3 would appear 
to warrant careful consideration for this purpose. 




RESTRICTED 


Chapter 13 

RADAR BOMBING ASSESSMENT AND TRAINING 


In the previous chapters of Part II, the necessity of 
providing highly skilled operators for airborne radar 
bombing systems was stressed many times. The de- 
sired degree of operator skill is developed only as the 
result of much training and experience over a period 
of many months. In order to reduce the training time 
and to lighten the operator’s task, the trend of bomb- 
ing computer design is toward the more complex 
mechanisms that function almost automatically. Al- 
though this policy succeeds in reducing the amount 
of operator training required, it results in a compen- 
sating increase of emphasis in training maintenance 
personnel. It thus becomes apparent that training, in 
one guise or another, will always play a major role in 
scientific warfare. 

First, the problem of bomb scoring and assessment 
will be considered. This includes all methods of de- 
termining the accuracy with which bombs are 
dropped, both in operational use and in training pro- 
grams. 

13.1 BOMB SCORING AND 

ASSESSMENT 

13.1.1 Introduction 

There are three general methods of determining the 
accuracy of bombing systems. The first of these is the 
dropping of practice bombs and observing their point 
of impact. Over the ocean or on bombing ranges this 
is generally a very satisfactory procedure. Of course, 
if damaged bombs are employed, this may not be the 
case. However, in training operators how to bomb 
complex industrial targets, the use of practice bombs 
would most certainly have severe repercussions. To 
avoid this, optical and radar pseudo-bombing 
schemes were devised. In these schemes, the impact 
point is determined from the velocity of the aircraft 
and the bomb characteristics, once the release point 
has been established. 

In developing a system for assessing bombing re- 
sults without dropping bombs, a number of require- 
ments of different relative importance have to be 
met. A list of typical requirements follows. 

1. Accurate determination of the impact point 
(within ±10 mils if possible). 

2. Operation over a large range of altitude, e.g., 
from 100 to 35,000 ft. 


3. Independence of weather conditions. 

4. Ease of operation. 

5. Simplicity of equipment construction. 

6. Ability to handle large numbers of planes in a 
short time. 

7. Portability of equipment. 

Of these requirements, the first is the most im- 
portant. It should be noted, however, that just as a 
micrometer is not used to measure the lumber in 
constructing a house, the necessary accuracy of a 
bomb scoring system depends on the accuracy of 
bombing systems being assessed. 

13.1.2 Optical Photoscoring 
Methods 

There are two possibilities for optical bomb scor- 
ing, namely, photography from the airplane being 
scored and photography from the ground (photo- 
theodolite). Of these, the latter is unquestionably 
more accurate, but it is restricted to only one target 
at a time and presents greater ground-air liaison 
problems. On the other hand, the method employing 
a vertical camera in the aircraft is not restricted to 
any particular target, but appreciable errors are in- 
troduced if the camera is not exactly leveled. Obvi- 
ously, both methods are subject to visibility restric- 
tions. 

Aircraft Camera Method 

The aircraft camera method is described in a report 
of the Department of Training and Operations of the 
Victorville Army Air Field. 72 A series of vertical aerial 
photographs are taken on the bomb run, and the 
camera is so arranged that a watch with a sweep 
second hand is photographed at the same time. The 
camera must be accurately leveled and oriented with 
respect to the longitudinal axis of the airplane. Type 
K8AB, K-21, and K-22 cameras can all be used for 
this purpose. 

The method was initially devised for scoring op- 
erators using the semisynchronous, radar-Norden 
bombsight tie-in (see Section 8.3.1). In this case, the 
bombardier takes pictures at five points on the bomb- 
ing runs, namely, (1) at the final radar check point, 
(2) at the bomb release point, (3) at a point approxi- 
mately midway between release point and impact 
point, (4) at the impact point (when the Norden 


155 


156 


RADAR BOMBING ASSESSMENT AND TRAINING 


sighting angle index reads zero), and (5) at a point 50 
sec in time from the release point. It is imperative 
that the airplane fly a straight line course from the 
beginning of the run until the last picture is taken. 

From these photographs it is not only possible to 
determine where the bombs would have fallen, but it 
is also possible to determine how the error was di- 
vided among such things as course errors, altitude 
errors, and bombsight error. 72 

This method of bomb scoring was used on a mass 
production basis at Victorville AAF for many 
months. The average photo scoring error encountered 
was stated to be considerably under 100 ft from 
10,000 ft altitude. 

Photography from the Ground 

Introduction. The photo-theodolite method as de- 
veloped at the Radiation Laboratory 23 fulfills require- 
ments 1, 4, 5, and 7 of Section 13.1.1 with varying de- 
grees of success. The major advantage of this method 
is the accuracy obtainable, which is considerably 
better than that of other methods now available, 
such as radar tracking, or photography from the air- 
plane. Radar tracking is not very satisfactory for 
handling runs at low altitudes (around 500 ft). 

Another advantage of the method is the relative 
simplicity of the equipment. No special optical sys- 
tems are needed — a standard camera is used, and 
the most delicate adjustment required is easily ac- 
complished with the aid of a plumb bob. The ground 
installation, exclusive of the communication equip- 
ment, weighs about 500 lb, but no single part weighs 
more than 100 lb. A radio transmitter and receiver for 
communication with the airplane are also essen- 
tial. 

The ground photography method has some definite 
limitations, the most fundamental of which is the 
need for clear photographic visibility. In addition, 
the maximum bombing error that can be measured 
is limited by the narrow angle of view of the camera. 
Thus, the error may be so large that the bombing air- 
plane does not come into the field of view of the 
camera. This limitation may be modified by another 
choice of camera or lens. A third restriction is the 
small number of bombing planes that one photo-the- 
odolite can handle concurrently. In a training pro- 
gram, the last restriction is troublesome. 

Description of Equipment. In bomb scoring by pho- 
tography from the ground, a special vertical photo- 
theodolite is used. The photo- theodolite consists of a 
standard 16-mm movie camera which photographs 


the bombing airplane through a set of wire cross hairs 
held about 54 in. above the camera. A clock capable 
of being read to 0.01 sec is photographed along with 
the cross hairs and the airplane. A 1,000-cycle note 
broadcast from the airplane is turned off at the bomb 
release point, and a special circuit on the ground, 
actuated by cessation of the note, starts the clock. 
By using very fast film and small camera stop open- 
ings, both clock and airplane can be kept in focus. 

Method of Operation. The photo-theodolite (see 
Figure 1) is usually set up as near to the center of the 
target as possible, although large displacements are 
permissible provided corrections are made in the 
final data. The intersection of a set of cross hairs is 
placed vertically over the center of the camera lens 
by using a plumb bob. While the airplane makes a 
bombing run, it is under the control of the automatic 
pilot and thus is held on the same course after the re- 
lease point is reached and until it passes over the 
target. (In Figure 1, the release point is at A.) When 
the plane appears in the camera finder, the camera is 
turned on and a movie is made which shows the 
airplane, the set of stationary cross hairs, and a clock 
which has already been started automatically by a 
signal from the plane at the release point. 

Method of Analyzing Photographs. When an analy- 
sis of the photographs is made, the frame is selected 
which shows the airplane (the center of the bomb bay 
is taken as the reference point for measuring dis- 
tances) at its closest approach to the intersection of 
the cross hairs. On this frame, the number of seconds 
since the release point is read from the clock. The 
difference between the clock reading and the known 
time of fall of the bomb, when multiplied by the 
ground speed of the airplane, gives the distance from 
the theodolite to the point of impact of an idealized 
bomb (range error). Such a bomb would have zero 
trail and would remain directly under the airplane as 
it fell (see Chapter 6) . The effect of trail can be com- 
puted, if desired, but zero trail bombs are entirely 
satisfactory in scoring bombing systems. 

The azimuth error is determined by measuring the 
distance of closest approach of the airplane course to 
the intersection of cross hairs on the film and con- 
verting to the actual distance expressed either in feet 
or in mils. 

Sources of Error. Of the possible errors involved in 
this method of assessment, one group is associated 
with the test equipment. Careful tests of the clock 
and its associated circuits show that the probable 
error in overall timing is not more than 0.02 sec. For 



BOMB SCORING AND ASSESSMENT 


157 


an airplane (or bomb) moving at 180 mph, 0.02 sec 
corresponds to about 5 ft in range. 

Errors in establishing the vertical depend on the 
care with which the intersection of cross hairs is 
placed over the center of the camera lens and the 
certainty with which this relation is maintained. By 
using a plumb bob, the lateral displacement between 
the center of the lens and the cross hairs can be held 
to less than *4$ in. This means that the vertical is 
established with a radial error of not more than 
±0.75 mil. Similarly any lateral displacement 
caused by the hair mount is smaller than % 2 in* 

The second group of errors is related to the ac- 
curacy with which the plane follows the same course 
after the release point as before. This error is not 
clearly known. The most definite statement that can 
be made about it is that, on the basis of the bomb- 
ing tests which have been made with this system, it 
appears that the probable deviation of the plane 
from its course is slightly, if at all, greater than the 
probable deviation of the bomb from its predicted 
course. A more complete description of this photo- 
graphic method may be found in reference 23. 

13.1.3 Bomb Scoring by Use of 
Ground Radar Systems 

Introduction 

A system consisting of the SCR-584 radar and the 
associated RC-294 automatic plotting table has been 
successfully used for bombing assessment. The SCR- 
584 (see Chapter 11) is a radar originally designed to 
furnish the position coordinates of a flying aircraft 
to a gun director for the control of antiaircraft fire. 
When used for bomb scoring, it is connected to the 
RC-294 plotting device which presents a ground plan 
view of the flight-path of the aircraft. The plotting 
scale is 1,000 yards per inch and the maximum 
plotting radius, 20,000 yd. The information derived 
from the timed ground track, together with the alti- 
tude, wind, and the bomb ballistics, enables the crew 
of observers on the ground to determine with great 
accuracy the impact point where bombs would have 
struck the ground if they had been released from the 
aircraft at a specified time. 

The advantages of the system are: 

1. It is independent of weather conditions which 
would render visual methods, particularly those in- 
volving vertical photography or sighting, useless. 
Training by this method can be carried out in any 
weather in which aircraft can fly. 


2. The accuracy of the instantaneous position data 
obtained with the system is superior to that provided 
by aerial vertical photographic methods, most of 
which have large inherent errors because of the diffi- 
culty and uncertainty of camera leveling. Further- 
more, the continuous nature of the information per- 
mits a more accurate determination of speed than is 
possible with discontinuous information, such as is 
obtained from strip photographs. The circular prob- 
able error [CE] in determination of the impact point 
is about 100 ft. 

3. By virtue of the data recording system used, 
the impact point computation and the bombing error 
can be determined much more easily and in a shorter 
time than by photographic methods. The routine 
procedure requires only 30 sec after the release time 
for computing and radioing the information back to 
the aircraft. 

4. The technique makes it possible to solve the 
bombing problem completely without the use of data 
from the aircraft other than the release-point time. 
This is particularly advantageous in training opera- 
tions, because it provides an overall check upon the 
data obtained and used by the bombardier in the air- 
craft during the bombing run. Moreover, by com- 
paring the position and ground track of the airplane, 
as obtained by the radar system at several points, 
with corresponding information from the airplane 
radar operator and bombardier, it is possible to re- 
solve the whole bombing error into its several com- 
ponents. Most photographic scoring methods utilize 
data obtained by the bombardier, and are therefore 
not independent of errors he may make or of instru- 
mental errors in the equipment in the aircraft. 

Several other methods for using the data derived 
from the SCR-584 for bombing assessment have been 
devised. These include (1) timed intermittent photog- 
raphy, or moving picture photography of the slant 
range, elevation and azimuth dials of a “data-box” 
attached to the SCR-584 data output selsyn system; 
(2) calibrated recording voltmeters attached to the 
data output potentiometer system; (3) manual plot- 
ting boards; and (4) other types of automatic plot- 
ting boards, for example, the MC-627 plotter (see 
Chapter 11). The RC-294 has the advantage over all 
photographic methods, including (1) above, of being 
much more rapid in operation and requiring no tedi- 
ous manual plotting of the data. It is more accurate 
than the recording voltmeter system and it is simpler 
to calibrate, operate and maintain than the MC-627 
plotter. 


% 



158 


RADAR BOMBING ASSESSMENT AND TRAINING 



/ 


< 


s' 


/ 



Figure 1 . Vertical theodolite in use beside a corner reflector. 


\ 


RESTRICTED 



BOMB SCORING AND ASSESSMENT 


159 


Equipment 

A block diagram of the SCR-584 and RC-294 
plotting system is shown in Figure 2 and a sketch of 
a typical field installation in Figure 3. The SCR-584 
provides azimuth and range data from 1/1 to 16/1 
speed selsyn generators attached, in the case of 
azimuth, to the azimuth gearing of the antenna 
mount, and in the case of range, to the SCR-584 
Ground Range- Altitude Converter Data Unit which 
has been modified to provide 1/1 and 16/1 selsyn 
output (see Figure 2). 

The azimuth and ground range data from the SCR- 
584 feed into two similar, separate servo-channels in 
the RC-294 , one for azimuth and the other for ground 
range. A close-up view of the selsyn and drive-motor 
assemblies of the RC-294 plotting-table mechanism 
is shown in Figure 4, and a simplified schematic dia- 
gram of one servo-channel in Figure 5. The channels 
operate as position servos and determine the azimuth 
position of a horizontal boom and the radial position 
of a small “range-cart” that rides on the boom. The 
boom and range cart establish the position of the pen 
which writes on the under surface of the tightly 
stretched plotting paper (see Figure 4). The top of 
the plotting paper is left free for the use of drafting 
instruments required for computing the bomb im- 
pact point. Instead of making continuous contact 
with the plotting paper, the plotting pen strikes it at 
one second intervals so as to provide a record of the 
time. Every tenth dot is extended to form a dash for 
ease in counting (see Figure 6) . The relay which oper- 
ates the pen is continually actuated by a clock cir- 
cuit, but may also be operated by a 1,000-c tone re- 
ceived via the radio from the bombing airplane. 

High frequency [HF] and very high frequency 
[VHF] transmitters and receivers are provided for 
ground-to-air communication, and there is an inter- 
communication system between the SCR-584. and 
RC-294 vans. 

Altitude information on the aircraft being tracked 
can be transmitted to the RC-294 from the SCR-584, 
where it appears on the dial of a selsyn repeater when 
the SCR-584 Ground Range- Altitude Converter sys- 
tem is switched to the altitude position. It is neces- 
sary to interrupt the ground track for about 10 sec to 
obtain an altitude reading in this manner. 

A description of the Radiation Laboratory radar 
bombing training [RBT] plotting system may be 
found in reference 20. The RC-294, which is based 
on the RBT system, is fully described 94 with com- 
plete installation, operation, and maintenance pro- 

* 


cedures. A redesigned version of the RC-294, desig- 
nated as the RC-310, embodies a number of improve- 
ments in mechanical design but is otherwise identical. 

Unless very high accuracy is desired, no special 
equipment is required in the airplane for bomb scor- 
ing with the RC-294. The so-called voice procedure 
requires the bombardier to call out the instant of 
bomb release over the aircraft radio so that the 
plotting table crew may mark the point upon the 
ground track. Timing errors up to 1 sec, correspond- 
ing to 440 ft at 300 mph, are inherent in this pro- 
cedure because of the slowness of human response. 

A more precise method of marking the release 
point is afforded by the release-point indicator 
AN/ARA-17. This device is connected directly to 
the Norden bombsight so that, when the bombsight 
indicates the bomb release point, the airplane trans- 
mitter is turned on and sends out a 1,000-c tone for 
a period of 1.5 sec when triggered by the release im- 
pulse. As previously mentioned, the 1,000-c tone 
automatically operates the plotting-pen relay and 
marks the plotting paper for the duration of the tone. 
The total overall time delay from release impulse to 
marking of the paper is less than 0.1 sec, which repre- 
sents only 44 ft at 300 mph. The release-point indi- 
cator 32 illustrated in Figure 7 A and B operates on 
24 volts d-c and weighs 9 lb. 

Operation 

The SCR-584 and RC-294 system is usually lo- 
cated about 10,000 yards from the major target in 
the target city, so that bombing runs from any direc- 
tion may be recorded on the plotting paper. In pre- 
paring the system for operation, there are two im- 
portant problems to be solved, aside from the 
technical operation. These are (1) obtaining the range 
of the target or targets from the SCR-584; and 
(2) orienting the SCR-584 or the plotting-table boom 
so that the position of the target on the plotting sur- 
face corresponds to the actual position of the target 
with respect to the SCR-584. In the exceptional 
cases, where the target is visible from the position of 
the SCR-584 and, in addition, provides an unambigu- 
ous radar echo, the positioning and orientation prob- 
lems are simple. The SCR-584 is merely locked on 
to the target, and the corresponding position of the 
plotting pen indicates the proper location of the tar- 
get on the plotting surface. In cases where the target 
is visible, but does not provide a suitable radar echo, 
the telescope on the SCR-584 antenna mount pro- 
vides a means for obtaining the bearing or azimuth of 


160 


RADAR BOMBING ASSESSMENT AND TRAINING 



Figure 2. Block diagram of SCR-584 with RC-294 plotting table. 


the target; but the ground range of the target must 
be obtained from scale maps, survey data, or other 
means, and set in to the range tracking unit manually. 

When the target is not visible from the SCR-584, 
it is necessary to resort to standard artillery pro- 
cedure for obtaining the range and bearing of the tar- 
get. A direct method of high accuracy is to carry 
aloft, over the target, a radar corner reflector sus- 
pended from a meteorological balloon whose position 
is accurately controllable by means of three guy 
ropes. The SCR-584 is “locked on” to the corner re- 
flector and its position marked directly on the plot- 
ting table. This method is accurate to within a few 
feet providing care is exercised by the crew of the 
SCR-584 in calibrating the entire radar system. 

Once the target positions are known, they may be 
reset into each new plotting sheet by means of range 
and azimuth dial readings in the SCR-584 or by a 


Plexiglas template. As soon as the target has been 
marked upon the plotting surface, the system is 
ready for operation. 

As discussed in Chapter 6, the velocity and direc- 
tion of the wind must be obtained in order to get a 
complete solution of the bombing problem. This is 
done by having the aircraft fly a wind triangle when 
it arrives in the range area for bombing practice. In 
this operation, the aircraft is guided by the plotting- 
table crew around a triangular course of approxi- 
mately 2 minutes on each side. A specified constant 
airspeed is maintained on the straight portion of each 
leg of the triangle. A typical course is illustrated in 
Figure 8. After the wind triangle is completed, the 
aircraft is released and goes out to its first initial 
point [IP] to begin the bombing practice. 

Figure 8 illustrates the fact that when the true air- 
speed vector, V a , is added at any angle to the wind 


% 


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BOMB SCORING AND ASSESSMENT 


161 



Figure 3. Overall view of RC-294 plotting equipment. 


vector W the locus of the vector sum will be a circle 
of radius | V a | centered about the head of the wind 
vector. The sum will always be the ground-speed 
vector V g and for any given heading will be measured 
from the tail of the wind vector to the head of the air- 
speed vector. For example, the airspeed vector OA is 
equal to the sum of the wind vector W and the 
ground-speed vector PA. 

The inverse of this process is used to find W and 
V a from the three ground-speed vectors obtained on 
the three legs of the wind triangle, as shown in 
Figure 8. To obtain the maximum variation in length 
of the three ground-speed vectors, they should be ap- 
proximately 120 degrees apart, although any angles 
can be used. They are drawn to scale from a common 
point and a circle is circumscribed about their heads. 
The radius of the circle is the true airspeed, | V ffl |, 
and OP is the wind vector. The accuracy of the de- 
termination depends upon the care exercised in 
measuring the ground-speed vectors, the ability of 
the pilot to hold a steady course, the constant air- 


speed along the sides of the wind triangle, and the 
precision of the geometrical construction. Wind and 
true airspeed data, obtained in this way, are usually 
accurate to better than 5 mph. 

The typical procedure followed during the bombing 
run consists of observing the aircraft on the plan 
position indicator [PPI] of the SCR-584 (out to 
70,000 yd, maximum) until it comes within auto- 
matic tracking range (32,000 yd), after which the 
tracking is switched to automatic. Just before the 
aircraft comes within 20,000 yd (maximum plotting 
range), the RC-294 crew makes an altitude reading. 
As the aircraft nears the estimated release point, the 
automatic release-point circuit is switched on and 
when the 1,000-c release signal is received, the re- 
lease point is automatically marked on the plotting 
surface, as illustrated in Figure 6. Once the release 
point has been marked, theaircraft immediately turns 
off the bombing course; and the SCR-584 may start 
tracking a second aircraft on its bombing run while 
the first aircraft is preparing to make another run. 


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RADAR BOMBING ASSESSMENT AND TRAINING 


If the plotting crew is skilled in traffic control and 
the initial point is sufficiently distant, several aircraft 
may be handled and a release may be made as often 
as once every 5 minutes. Normally, however, only 
one or two planes are controlled simultaneously and 
a score is made about every 10 minutes. It must be 
emphasized here that the problem of traffic control 
and identification of the aircraft making the bomb- 
ing runs presents a major difficulty in the operation 
of the RC-294 system (or of any other ground-based 
system). Identification, when a number of airplanes 
are being monitored, is usually accomplished by 
directing the pilot to make an identifying 90-degree 
turn, and observing this maneuver on the PPI or 
plotting table. For a large-scale training program, 
either an auxiliary traffic-control radar, a direction 
finder or the use of beacons, would aid materially in 
solving the identification and in increasing the traffic 
handling capacity of the system. 

The geometry of the bombing problem is shown in 
Figure 9 (see also Chapter 6) . The general discussion 


or solution of the bombing problem from ground 
based radar systems is presented in Chapter 11. A 
typical ground track with the bomb release point 
marked on it is shown in Figure 6. The ground speed 
| | may be found by measuring the distance 

covered in a given number of seconds and using an 
appropriate conversion factor. In Figure 9, AC is 
equal to that length of ground track which corre- 
sponds to the number of second marks equal to the 
time of fall of the bomb. The construction establish- 
ing the impact point, E, may be completed from 
wind and bomb ballistics data. The techniques of 
calibration, orientation, and plotting of the RC-294 
system are described in reference 31. 

An extensive program of practice bombing error 
analysis was set up by the Army Air Forces during 
the last 6 months of World War II. The analysis was 
made possible by information derived from various 
scoring methods such as the RC-294 and Camera 
Bombing (Section 13.1.2, “ Aircraft Camera 

Method”). 



B—Bi 


PLOTTING BOOM K PEN CARRIAGE 


PAPER ROLL 


TABLE LIGHTS 


LAMP BALLAST 


ACCESS DOOR 


ACCESS DOOR 


Figure 4. View of plotting table with paper removed. 


CABLE I 


CABLE A^^HAZiMUTH DRIVE UNIT 


LAMP BALLAST 


V 



BOMB SCORING AND ASSESSMENT 


163 


Accuracy 

The major sources of error inherent in the SCR- 
584 and RC-294 plotting system are a function of the 
distance of the SCR-584 from the target, the altitude 
# and airspeed of the aircraft, and the care exercised by 
the plotting crew in operating the system and com- 
puting the impact point. If the speed, altitude, or 
heading of the aircraft is varied appreciably during 
the portion of the bombing run over which the 
measurements are taken, an unknown amount of 
error will be introduced into the result. However, 
such a bombing run would be worthless from the 
standpoint of the operation of the bombsight in the 
aircraft and would normally be disregarded. 

The following tables illustrate the major sources 
of error and the probable errors (PE) to be expected 
from them in the specific case of an aircraft flying at 
200 mph true airspeed, 23,000 ft altitude, at right 
angles to a 50-mph crosswind. The time of fall is 
taken as 40 sec, and the target is assumed to be 
about 10,000 yd from the SCR-584. 


Table 1. Typical probable errors (PE) in determina- 
tion of bomb release point using RC-294 system. 


PE Yards 

In orientation of the SCR-584 (£ mil) 5 

In aligning telescope and radar axis (| m il) 5 

In slant range from SCR-584 (tracking error) 15 

In plotting due to azimuth tracking error (1.6 mils) 16 
In plotting due to elevation tracking angle error 
(1.6 mils) 11 

In triangle solver (SCR-584 data unit) 8 

In release point mechanism (after subtracting fixed 
delay) ' 5 

In reading dot position 3 


(2 PE 2 )* = Total PE in position of release point with 

respect to target 28 


Table 2. Typical probable errors (PE) in computing 
the impact point using the RC-294 system. 

PE Yards 

In extrapolation of ground track (j°) 16 

Introduced by timing clock (0.2% of V g -t / ) 8 

In computing cross trail (wind and airspeed determi- 
nation) 5 

In time of fall ( + 30 yd error in altitude) 10 

Total PE in computation of impact point 21 



Figure 5. Servo block diagram. 





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RADAR BOMBING ASSESSMENT AND TRAINING 


The resultant probable error in impact point de- 
termination with respect to the target is 35 yd. These 
accuracy figures have been checked, roughly, by 
photographic methods at the Radiation Laboratory 
and by bombing trials at the AAF Proving Ground 
in Florida. 

It is clear from Tables 1 and 2 that the major 
source of error is the SCR-584 tracking system. A 
new version of the SCR-584 known as SCR-584X 
incorporates a tracking system with probable error 
only one-third that of the SCR-584. The SCR-584X 
would be the logical substitute for the SCR-584, in 
applications where greater accuracy than that 
quoted above is desired. 


best method for obtaining experimentally such items 
as the complete trajectories of aircraft under test, 
and bomb ballistic data. 

13.1.4 Bombing Assessment from 
PPI Photographs 

The assessment of bombing accuracy from PPI 
scope photographs presents a number of interesting 
features despite the limitations of the method. In 
H2X bombing, it is necessary to take a series of 
scope photos during the bomb run, preferably spaced 
equally in time and at 10-second or shorter intervals. 


RELEASE POINT TRACK 


RELEASE 

POINT 

Figure 6. Ground track. 

Other Radar Bomb Scoring Methods Employing 
the SCR-584 

The RC-305 manual plotting system is an interim 
system developed as a substitute for the RC-294. 
The operation of the system is identical with that 
of the RC-294 system, except that the data is manu- 
ally recorded from dial readings at 10-sec intervals 
and then plotted on a scale of 1,000 yd per inch. Be- 
cause of the discontinuous nature of the data and the 
introduction of the human element, the operational 
probable error in determining the impact point is 
about 125 yd. The RC-305 is fully described in a 
Radiation Laboratory report. 36 

A recording voltmeter system has been proposed 
as another interim substitute for the RC-294. This 
system utilizes the slant range, elevation cosine, and 
azimuth sine and cosine potentiometers in the SCR- 
584 and presents on the recording voltmeter, volt- 
ages corresponding to the X and Y components of 
ground range. The recorded data are then manually 
transcribed to a ground track plot as in the case of 
the RC-305. The estimated probable error of this 
system is about 100 yd. 

Probably the most accurate scoring system em- 
bodying the SCR-584 is one in which the data output 
dials of the SCR-584 are photographed at 1-sec 
intervals or less, and the data are later carefully 
plotted manually. Because of the time element in- 
volved, this method is not as convenient to use for 
training as the RC-294 system. It is probably the 



. 

Figure 7 A. Top view of release-point indicator. 



Figure 7B. Front view of release-point indicator. 



BOMB SCORING AND ASSESSMENT 


165 




Figure 9. Bombing diagram. 

The instant of bomb release should be indicated by 
a signal light in the corner of the picture or some 
equivalent method. 

Plotting of the location of the successive photos 
on a suitable map will enable one to determine course, 
ground speed, and location of bomb release, from 
which impact can be determined. The series of pic- 
tures should include several after the time of bomb 
release. 

Estimates of the accuracy of this method of de- 
termining impact point run from one-quarter mile to 
a mile or more. One-quarter mile is probably opti- 
mistic, even under the best conditions. A full rota- 
tion of the scanner requires 3 sec and the aircraft 
travels approximately one-fifth of a mile in this 
time. With everything else perfect, the uncertainty 
of location of a single picture (3-sec time exposure) 
must then be one-fifth of a mile. On the other hand, 
the upper limit of accuracy depends upon accurate 
plotting of each photograph. In general, for photo- 
graphs taken as the bomb release point is approached, 
plotting becomes more difficult, since the operator 


usually has expanded the scope to some unknown 
radius, removed the range marks, and readjusted the 
receiver gain. This difficulty can be partially over- 
come by the practice of restoring the scope to a pre- 
determined adjustment, including range marks, just 
prior to bomb release. 

Even granting that the accuracy of the method is 
probably no better than one-half mile, there are 
several reasons for using it. The uncertainty is free 
from bias with perhaps one exception — if the H2X 
set is performing very badly, the photographs will be 
unsatisfactory and the bomb run will also be poor. 
On the other hand, assessment by strike photos or 
photo reconnaissance is likely to be very biased. 

If photo reconnaissance is used, bomb craters well 
away from the target, especially in open areas, are 
easily seen. On the other hand, craters near the target 
may be difficult or impossible to locate. This is par- 
ticularly true when the target is an industrial plant 
made up of similar buildings. 

If photo reconnaissance is discounted, as was done 
by the Operational Analysis Section of 8th Air Force, 
and strike photos are used, we are faced with another 
bias. Planes flying wide of the target are likely to get 
recognizable pictures of the ground, while those flying 
directly over the target will be more likely to find the 
ground completely obscured by haze and smoke 
from the target itself. Therefore, the most satis- 
factory strike photos will show large bombing errors. 

Entirely aside from the question of accuracy, it 
was found at 8th Air Force that assessments based 
on scope photos and made by a special section of 
the Photo Wing could be in the hands of the oper- 
ating squadrons within two days of the mission ; the 
results from the more careful analysis of strike photos 
were not completed until 3 or more months later. By 
this time, most of the value for improvement in tech- 
nique has been lost, as frequently the bomber crews 
concerned had returned to the United States. 

In this connection, the use of movie cameras should 
be mentioned. In addition to the above possibilities 
for assessment, the continuity of the projected movie 
print presents information on the entire mission that 
cannot be obtained in any other way. It is possible 
to assess the execution of the rendezvous, cross- 
country navigation, turning at initial point, and ex- 
ecution of the bomb run. Such difficulties as killing 
drift and departures from briefed courses in navi- 
gation can be seen at once. With proper facilities, a 
print can be projected at a squadron critique within 
2 hours of landing time. These extra advantages are 


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RADAR BOMBING ASSESSMENT AND TRAINING 


very valuable in improving the overall of teamwork 
between radar operator, navigator, bombardier, and 
pilot. 

Evaluation of the accuracy of a Micro-H mission 
from scope photographs is not as easy as for an H2X 
mission. This is true because only the beacon signals 
are visible on the PPI photographs. The general 
execution of navigation can be assessed by an exami- 
nation of a series of photographs, or better yet, a 
viewing of a movie film made up of successive time 
exposures of the scope. Unless an additional movie 
of the computer drum settings is made, no very good 
evaluation of impact point can be made. Errors in 
range computation, errors in calibration, errors caused 
by malfunctioning of the range unit, and errors re- 
sulting from careless setting of the computer drum, 
will not show up on photographs of the scope. Motion 
pictures of Micro-H missions, however, would permit 
a general evaluation of the smoothness of execution 
of the mission which would be of considerable value 
in pointing out and correcting mistakes of inex- 
perienced crews. 

13.2 TRAINING 

13.2.1 Introduction 

The large-scale introduction of any new equipment 
into the Armed Forces always imposes the formidable 
task of training the personnel to utilize it properly. 
The purpose of this section is to outline some of the 
problems encountered in radar training during World 
War II and to describe their solutions together with 
some recommendations for future peacetime training 
procedure. Training for microwave airborne radar 
bombing will be used as an example of the complex 
and dRerse problems that arise in all phases of radar 
training including aircraft to surface vessel and fire 
control. 

A radar bombing mission can be divided into two 
parts — planning and execution. Planning implies 
that the headquarters staff has been trained suffi- 
ciently to recognize the capability and limitations of 
the radar equipment. Adequate radar reconnaissance 
and analysis personnel should also be located at 
headquarters to permit selection of targets suitable 
for radar attack. Moreover, intelligence officers who 
brief the combat crews should be well-trained in all 
aspects of radar navigation and target identification. 

Execution of the mission necessitates, first of all, 
that the equipment be operational, that is, it must 


have been properly installed and its technical per- 
formance must be up to a certain satisfactory stand- 
ard. This involves the training of installation and 
maintenance technicians and establishment of rou- 
tine procedures for assessment of the performance of 
the equipment. The operators (enlisted men and 
officers) who use the equipment during the mission 
require training in operating the system, in radar 
navigation, in target identification, and in radar 
bombing. The combat crews need practice in oper- 
ating together as teams. 

In addition to the headquarters personnel directly 
connected with planning and execution of the mis- 
sion, other sections are required, such as a bombing 
assessment and analysis group which must be trained 
to evaluate bombing performance. A theater training 
group must also be available to devise and supply 
any additional operator and crew training required. 

13.2.2 Operator Training 

One of the first schools in the United States for 
training in the techniques of radar bombing of over- 
land targets was located at Grenier Field in Man- 
chester, New Hampshire. There, operators and crews 
of the 812th Bomb Squadron were taught how to 
use the (initial preproduction) H2X equipment. 
These crews were destined to lead the 8th Air Force 
as pathfinders during the winter of 1943-1944. Since 
no Army personnel had ever been trained in the use 
of this equipment, there were no available Army in- 
structors; consequently the designers at the Radi- 
ation Laboratory acted as instructors. These men 
were well acquainted with the technical features of 
the equipment, but at the time, were unaware of 
some of its serious limitations. Neither were they 
completely familiar with the bombing problem. How- 
ever, the navigators and bombardiers, who started 
learning how to operate the radar equipment, sup- 
plied the needed knowledge and readily worked out 
a radar bombing technique. A partially successful 
effort was made to assess the resulting bombing per- 
formance by photographing the impact point through 
a Norden bombsight synchronized with the radar. 

After a short period of training, the crews flew to 
the European Theater to start operations. In the 
interests of speedily supplementing the original 
twelve H2X crews in the theater, the training pat- 
tern established by the original group at Grenier 
Field was abridged to such an extent that the new 
crews who went into combat theaters from the 


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167 


United States had too little training. As a result, 
large theater training programs had to be set up 
before the most effective use could be made of the 
equipment. Because of the difficulties of training 
under theater conditions, it was never possible to 
completely realize the instrumental capability of this 
radar equipment in the European Theater. 

The experiences of the 8th Air Force, noted above, 
are typical of the early training programs in which 
the air forces were attempting to build a successful 
training program that would produce the best re- 
sults in the theater within the time limit imposed by 
expediency. 

By the end of the war, it appeared that an ade- 
quate operator training program should involve the 
following four major phases. 

1. Individual ground school training where the stu- 
dent learns the rudiments of navigation and bombing 
procedure, interphone technique, scope interpreta- 
tion, and operation of the controls of the radar set. 
He then utilizes his newly acquired knowledge on a 
supersonic trainer to obtain simulated bombing and 
navigation experience. (The supersonic trainer em- 
bodies a miniature scale model of the terrain im- 
mersed in a water tank which is scanned at supersonic 
frequencies by means of a crystal. The images pro- 
duced on the PPI scope are remarkably similar to 
those which would be seen on the radar when flown 
over the corresponding territory. 27 ) An important 
feature of the training program is competitive scor- 
ing of the student’s proficiency. It is also important 
to have instructors who are well qualified to teach 
their subject. In the interests of uniformity, the in- 
structors should be supplied with a specific lesson 
plan including briefing material in a form of an 
instructor’s manual, for example, the APQ-7-T1 
trainer published by the Training Aids Division. 

2. Advanced flight training where the student is 
flown over many different complex targets in order to 
gain proficiency in scope interpretation. The initial 
phase of this training consists of teaching the student 
the fundamentals of drift-killing, wind measurement, 
and operation of the set including computer manipu- 
lation. This is accomplished by running a few mis- 
sions on pin-point targets where scope interpretation 
is no problem. After the initial phase, the student is 
flown over complex targets to learn the technique of 
scope interpretation. 

The two vital elements of this program are proper 
briefing of the operator with scope photographs before 
the mission and scoring of his bombing performance. 


Since target identification has been consistently the 
largest source of error in radar bombing, an auto- 
matic scope camera is a very important accessory to 
this phase of the training program. It is axiomatic 
that the student will generally make little effort to 
correct his ways unless he is firmly convinced of his 
error. With the photographic record of the student’s 
individual performance before him, the instructor can 
easily convince the student of his errors, for example, 
in target identification or in failing to kill drift prop- 
erly. The effectiveness of scope cameras thus utilized 
was clearly demonstrated by the combat experience 
of the 315th Wing of the 20th AAF in monitoring the 
bombing procedure of their AN/APQ-7 radar oper- 
ators and effecting corrections to their techniques. 

3. Continuation training to as great an extent as 
possible while the student is in transit or waiting for 
operational duties at staging areas. In many in- 
stances, the long time interval between the end of 
formal training and the beginning of combat experi- 
ence results in the student forgetting practically all 
he has learned during the training courses. 

4. Theater training in which the student is sub- 
jected to refresher courses in the fundamentals, in- 
tensive synthetic training for the purpose of main- 
taining proficiency in scope interpretation and for 
studying new target areas, and such flight training as 
is required for modifying previous training ideas or 
introducing new ones to suit the peculiarities of the 
theater. In general, theater training has been found 
to be difficult. It is often hasty; bomb scoring and 
other evaluation methods are difficult to operate and 
maintain in the theater; and the normal tendency is 
for the operators to go into combat as soon as they 
show any signs of having mastered the technique, 
even though they may not have become reasonably 
proficient. Without question most of the training 
should be accomplished in the United States and 
theater training restricted to the bare minimum 
necessary for refresher training and for introducing 
techniques peculiar to the theater. 

13.2.3 Maintenance Training 

The operational success of a radar bombing device 
depends fundamentally upon two factors: (1) the 
equipment must perform reliably and accurately at 
all times, and (2) the combat operator must have 
complete confidence in its reliability, accuracy, and 
capability. Since good maintenance underlies the 
proper technical operation of the equipment, it would 


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RADAR BOMBING ASSESSMENT AND TRAINING 


appear that the success of the entire program depends 
critically upon the quality of the maintenance. It 
must be emphasized that the training of ground 
maintenance mechanics is as essential to obtaining 
good operational results with the equipment as is 
adequate operator training. This is particularly true 
of the more fully automatic computers such as GPI 
(Chapter 9). 

The purpose of the maintenance training program 
is to familiarize the maintenance mechanics com- 
pletely with (1) the system on which the measure- 
ments are to be made, (2) the reasons for making the 
measurements, (3) the test equipment, and (4) the 
operation of the test equipment. Besides a thorough 
instruction in the basic principles of operation of the 
radar system and associated equipment, the main- 
tenance personnel require sufficient operational ex- 
perience to enable them to perform the measurements 
with ease and facility and to make the indicated ad- 
justments or repairs. 

In addition to training on the technical aspects of 
operating, servicing, and maintaining the equipment 
and evaluating its performance, there is another 
phase of training on matters of supply, maintenance 
records, and standard operating procedures for main- 
tenance. 

These two types of training are required for per- 
sonnel in first, second, third, and fourth echelons of 
maintenance; in the supply corps which must become 
familiar with the new equipment in order to estimate, 
order, and stock supplies; and in the headquarters or- 
ganization which will be responsible for establishing 
the level of equipment maintenance. 

The division of maintenance personnel into the 
echelon category suggests a corresponding division 
of the training program, since personnel in the first 
category do not require the same skills, and conse- 
quently the same type of training, as those in other 
categories. 

Although most of the training activities can be 
handled entirely in the United States, the modifica- 
tions to equipment, that inevitably occur during 
wartime, always necessitate some theater training. 

In connection with maintenance, it cannot be too 
strongly recommended that the designers be allowed 
to become familiar with the maintenance problems 
that arise in the field and how such problems are 
handled, so that the equipment can be designed for 
easy maintenance. It is also desirable in this regard, 
that the Technical Training Commands in the United 
States become familiar with the equipment during 


the design stages so as to be in a good position to fore- 
see the training problems, the requirements for train- 
ing aids, and the requirements for maintenance 
mechanics to operate the equipment during the 
initial phases of the training program. 

A sketch of the organizational requirements for 
handling a typical maintenance training program is 
shown in Figure 10. The organization is headed by a 
representative group from the Technical Training 
Command. The function of this group is to set up the 
program at one of the training centers, and conse- 
quently it is essential that its members be thor- 
oughly trained through contact with the technical 
designers and Army tacticians during the design and 
procurement period. The function of the group of 
officers from the Air Technical Service Command is 
to become familiar with the equipment at an early 
date and to investigate and remedy the various de- 
ficiencies which inevitably show up in any new pro- 
gram. Other functions of the organization are to 
supply experts to supervise the initial training, to 
instruct local supervisors in maintenance of the 
equipment being used for the training, to carry on 
continuation instruction of the maintenance men 
during the period while the operator training groups 
are being trained in one of the Continental Air Forces, 
and to provide continuation training during initial 
phases of the combat operations until such times as 
the combat groups can carry on their radar main- 
tenance without further assistance. 

The sequence of instruction is of vital importance. 
If the men are trained before the equipment is ready, 
they will forget what they learned while they wait, 
and if they are trained too hurriedly the quality of 
maintenance will suffer. 

It can easily be appreciated that serious conse- 
quences will result if the number of men needed in the 
top blocks (Figure 10) were underestimated or if a 
miscalculation of the time required to train them 
were made. To avoid such an occurrence, it is vitally 
important that training of personnel in the top blocks 
begin well in advance of the installation program. 

Besides instruction in the principles of operation of 
the equipment, the maintenance team should have 
actual experience in and responsibility for perform- 
ing maintenance. For example, it is completely fea- 
sible that the men whose job will be to perform first 
and second echelon maintenance during combat oper- 
ations can be given the identical job during group 
training operations in the United States. Although 
this point may seem too obvious to be mentioned, it 


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TRAINING 


169 



Figure 10. Maintenance training. 


is a fact that this simple method of giving the main- 
tenance men experience prior to combat was not 
generally employed during the war. 

As in the case of radar operator training, in-transit 
training for maintenance men is highly desirable, par- 
ticularly if they are required to spend extended 
periods in staging or rest camp areas. 

Moreover, it must be emphasized that the assump- 
tion should never be made that men are qualified to 
perform maintenance simply because they have srtis- 
factorily completed the courses at a service training 
school. Additional requirements are that they must 
have obtained a backlog of practical experience by 
actually performing maintenance work and that the 
time interval between the end of their training and 
the beginning of maintenance work in the theater 
be small. 

An essential part of the maintenance training pro- 
gram is the training equipment. Since it often takes 
as long for radar training equipment to be developed, 
manufactured, and put into operation as it takes for 
the development and manufacture of the basic radar, 
it is obvious that the development and production of 


radar training aids must parallel that of the radar 
itself. This is another reason why the training com- 
mands should take an interest in the radar from its 
inception. In addition to training aids, instructor’s 
manuals, instruction and training manuals, instruc- 
tion course plans and literature, and trainers of vari- 
ous sorts, the training courses also require that the 
basic radars, spare parts, power supplies, and test 
equipment be available at the start of the training 
courses. Because of the nature of maintenance train- 
ing, it requires a larger ratio of both spare parts and 
test equipment than is necessary for combat oper- 
ation, for a student should be expected to make some 
mistakes which will damage the gear used for train- 
ing. Such mistakes are a natural part of his learning 
process and give a reasonable indication that he is 
doing something on his own initiative. 

With regard to the basic radar components, the 
first units produced should be allocated for training. 
It is so important that training start as soon as 
possible that it is usually worth while to have a 
limited number of systems built on a model-shop 
basis, prior to mass production, for employment in 


170 


RADAR BOMBING ASSESSMENT AND TRAINING 


the initial stages of the training program. This pro- 
cedure has the added advantage that flaws in design 
may be discovered and remedied before large-scale 
production gets under way. 

In allocating equipment for the theater, it is im- 
portant to send more sets than will actually be re- 
quired in combat installation so that equipments for 
theater training will be available. In emergencies, 
this additional equipment can also be used as a back- 
log of spare parts. 

A peacetime training program can, of course, be 
more orderly and thorough than one developed dur- 
ing the stress of wartime. Although the overall phi- 
losophy of maintenance training can be carried over 
into peacetime operation, some of the training prob- 
lems are changed. For example, it becomes easier to 
treat the principles of operation thoroughly but more 
difficult to provide the necessary experience in actu- 
ally performing maintenance on a large scale and 
under field conditions. This is an extremely serious 
handicap and very high standards of equipment per- 
formance must be established in order to keep the 
mechanics alert. 

In contrast to wartime operations, it should be 
possible to separate completely operator training and 
maintenance training so that the operator training 
program will not be hindered by partially trained 
maintenance men. 

13.2.4 Training of Staff Officers 

Given well-trained operators and mechanics, the 
success of the radar equipment depends on how in- 
telligently it is employed by the staff officers who do 
mission planning, prepare radar intelligence, and pro- 
vide briefing of the combat crews. This phase of the 
training program was almost completely neglected 
during World War II with the result that quality of 
radar intelligence and briefing varied from group 
to group depending upon the amount and quality of 
the theater training obtained on the spot. 

Some exceptions to the dearth of trained radar staff 
officers did exist. Certain groups were able to develop 
excellent briefing procedures that both interested 
and stimulated the combat crews. For example, in 
the 8th Air Force, a radar intelligence group did an 
excellent job of getting out briefing material such as 
radar maps and scope photographs of important tar- 
get areas. A genuine effort was made in this case to 
obtain good scope photographs and to catalog these 
in such a way that all group officers could be quickly 


and easily provided with the information. A similar 
program was under way in the 20th AAF several 
months before the end of World War II. As another 
example, the 315th Wing developed, by the end of 
the Japanese war, a unique and interesting briefing 
procedure utilizing fluorescent maps that proved 
highly successful. 

It is now recognized, however, that these theater 
training programs brought out under combat condi- 
tions were neither so complete nor so basic as a well 
organized program in the United States could have 
been. Some of the tentative requirements of such a 
program are outlined below. 

Because of the novelty of radar bombing in World 
War II, a specialist on radar bombing was not in- 
cluded in the regular table of organization of staff 
officers. As a result, planning of radar bombing mis- 
sions was often accomplished by a visual bombardier 
or some other person not fully aware of the capa- 
bilities and limitations of the radar. In contrast to 
planning for a visual bombing mission, in which the 
staff officers were supplied, by the intelligence group, 
with all types of maps marked with the latest infor- 
mation on the target and its surroundings, in the 
beginning radar bombing missions were often planned 
with only the aid of optical reconnaissance photo- 
graphs and ordinary maps. As a result the radar 
bombing missions frequently ended in failure. From 
this bitter experience, it was learned that not only 
are detailed photo-reconnaissance and target infor- 
mation from ground intelligence needed, but, also, 
complete radar reconnaissance information is re- 
quired so that the appearance of the radar images of 
the target and its approaches can be studied in detail 
and memorized by operators and bombardiers prior 
to the mission. This requirement makes it imperative 
that radar intelligence officers obtain scope photo- 
graphs well ahead of the mission so that the best 
radar approaches and distinctive landmarks for navi- 
gation and bombing can be determined. 

13.2.5 The Victorville Experiment 

Not until mid- 1944 was it realized to what extent 
the lack of operator training was affecting bombing 
performance in the theater. In an effort to investi- 
gate some of the basic features of training, the AAF 
established the “ Extended Training Experiment for 
H2X” for radar operators at Victorville Army Air 
Field. 75 

In these experiments a group of twenty graduates 


TRAINING 


of the standard AN/APQ-13 radar observer bom- 
bardment course were given considerable additional 
training to see what improvement could be achieved 
in their bombing performance. The synchronous 
bombing technique, in which the radar is synchron- 
ized with the Norden bombsight, was employed (see 
Section 8.3.1). The radar operators were teamed with 
returned combat bombardiers for the duration of the 
experiment. All runs on any single routine mission 
were flown on a single target selected from a total 
of eleven complex targets. Although the “ target 
familiarity ” resulting from making all runs on a 
single target was undesirable, this procedure was 
followed to enable the making of more runs per hour 
than would have been possible on different targets, 
and to enable the selecting of the target for good 
photographic weather in order to permit scoring. All 
runs were scored photographically as described in a 
Victorville Army Air Field publication 72 (see Sec- 
tion 13.1.2). Each scored run was studied by the 
student and his errors were analyzed into seven com- 
ponents: (1) drift error, (2) final point deflection 
error, (3) range error caused by altitude error, 
(4) radar range error, (5) final point bombsight range 
error, (6) ground-speed range error, and (7) range 
error due to time of fall error. Some of the results are 
given in Table 3. 


Table 3. Improvement in radar bombing as a function 
of hours of practice bombing. (Values corrected to 
altitude of 12,000 ft). 


Training 

period 

(hours) 

Circular proba- 
ble error (CE), 
all runs 
(feet) 

Circular proba- 
ble error, first run 
on each mission 
(feet) 

Special missions, 
circular proba- 
ble error 
(feet) 

0-25 

3,137 

4,283 


25-50 

2,028 

2,701 

2,590 

50-75 

1,885 

2,426 


75-100 

1,627 

1,829 

1,743 

100-125 

1,686 

2,010 


125-150 

1,543 

1,767 

2,825 


At the end of 50 hours, 100 hours, and 150 hours 
each student flew a special mission over unfamiliar 
territory and made one bomb run on each of four 
new targets in order to permit a study of the effect 
of target familiarity. On all of these runs the oper- 
ators were briefed with PPI photographs in the usual 
manner. The results of these special missions are 
shown in Column 4 of Table 3 and indicate that, 
except for the 150 hours trial, the circular probable 
error introduced by bombing totally unfamiliar tar- 
gets is no greater then the circular probable error of 


the first bombing run on previously bombed or 
familiar targets. (The large error in the 150 hours 
trial may have been due to the operator’s boredom 
and anxiety to finish the experiment and go home. 
This indication that training could be overdone re- 
sulted in the recommendation that the extended 
training program be limited to 100 hours of main 
scope time.) 

The analysis of scored runs confirmed the belief 
that inaccurate scope interpretation is the most seri- 
ous obstacle to accurate radar bombing. The follow- 
ing is a quotation from the Victorville Report. 72 “It 
is consistently found that the two largest sources of 
error are the final point deflection error and radar 
range error. Both of these indicate an inaccuracy in 
locating the precise aiming point. The importance of 
these two sources of error can be realized by finding 
the average circular error if all other types of error 
were zero. In the first 25 hours, for example, the 
theoretical circular error produced by the final point 
deflection error and radar range error alone is 3,005 ft. 
The average circular error obtained in the first period 
was 3,137 ft. Scope interpretation alone could ac- 
count for all the obtained CE. Similar results are ob- 
tained for other periods. This does not mean that the 
other sources of error are negligible. Because certain 
errors compensate for each other, the total of all the 
individual errors (neglecting algebraic signs) may be 
much greater than the total error. Nevertheless, the 
results indicate that there is sufficient error in scope 
interpretation alone to produce a CE practically 
equal to the empirically obtained CE!” 

As a consequence of the “extended training experi- 
ments” the next large Air Forces Training Program, 
on radar bombing, namely, operator training on 
AN/APQ-7 (Eagle) equipment in B-29’s, was ex- 
tended to some extent. This program included: 
(1) Four weeks (20 main scope hours) of initial oper- 
ator training at Boca Raton Army Air Base in a 
course similar to the basic AN/APQ-13 radar oper- 
ator’s course, including supersonic trainer work, prac- 
tice navigation, dry runs on complex targets, and 
practice bomb drops on point targets in the water 
with splash photography assessment. This last pro- 
cedure provided an evaluation of the student’s 
ability to bomb a point target. (This is a far simpler 
problem than that of bombing overland targets be- 
cause scope interpretation is reduced to observation 
of a single spot on the screen.) The scoring procedure 
also permitted an appraisal of the student’s ability in 
the fundamental process of killing drift and measur- 


172 


RADAR BOMBING ASSESSMENT AND TRAINING 


ing ground speed. (2) The student radar observer 
bombardiers then passed from Training Command 
into the Second Air Force for combat crew training 
which consisted of two and one-half weeks (26 hours 
of main scope time) for the 315th Wing and 5 weeks 
(61 hours main scope time) for the 316th Wing. The 
Training Command facilities at Victorville Army 
Air Field were utilized by the Second Air Force for 
this program. It is significant that, in this course, a 
great variety of complex targets were utilized and 
that the briefing by scope photographs was excellent. 
A photographic bomb scoring system provided 
assessment of a bombing performance and a basis for 
competition among the students. 


Table 4 



A 

315th Wing 
average 

B 

316th Wing 
average 

Training days 

15 

34 

Bomb runs per student 

27 

63 

Main scope hours per student 

26 

61 

Ground-speed error 

12.5 knots 

8.3 knots 

Drift error 

3.6° 

2.9° 

Circular probable error 

3,000 ft 

2,100 ft 


Table 4 indicates the overall performance of the 
students for the 315th and 316th Wings. 

Table 4 shows the value of the additional training 
on complex targets. In terms of bomb fall density, 



Figure 11. Progress curves, VAAF class 45E. 


i.e., circular probable error squared, there is a factor 
2:1 in the effectiveness of the operators of the two 
wings. This could be interpreted as meaning that 
one wing of “B” operators is worth two wings of 
“A” operators. 

The progress curve indicating the improvement in 
bombing of one of the classes is shown in Figure 11. 
The scores are divided into five periods of four mis- 
sions each and the average circular error in each 
period is plotted. The circular probable error of 3,000 
ft at the beginning of the course was reduced to ap- 
proximately 1,500 ft at the end of five weeks’ train- 
ing. In terms of bomb fall density the improvement 
is a factor of 4 and can be interpreted as meaning 
that one wing manned by these trained operators is 
four times as effective as a wing manned by operators 
without the extended training course. The cost of 
this training is of course insignificant compared with 
the cost of three additional B-29 groups. 

13.2.6 Conclusion 

Although the foregoing treatment of training is far 
from complete, it is hoped that some indication of the 
nature and importance of the problem has been 
given. In the opinion of the authors, the one factor 
that would have done most to improve the accuracy 
of radar blind bombing was increased skill (through 
training) of the operators. Whether the added time 
needed to produce this skill is more important than 
the early appearance of a large number of partially 
trained operators over the target is a matter for 
military tacticians to decide. 

The alternative approach is to design equipment 
that is nearly automatic in operation. In this case, 
the required degree of skill could be obtained in the 
proverbial “three easy lessons.” While this maybe 
possible for any future military use of radar equip- 
ment, it is also true that the majority of important 
war weapons initially appear in crude form and de- 
pend heavily on operator skill for their performance. 
In view of this, an increased flexibility in establishing 
new organizations within the military framework is 
desired. It would then be possible to establish special 
groups whose main purpose would be to obtain the 
maximum use of any new weapon by all possible 
means, which would include an adequate training 
program. 

Further information on specific training problems 
will be found in references 65-67, 73, and 74. 



PART III 


AIRCRAFT INTERCEPTION 




Chapter 14 

THE INTERCEPTION OF ENEMY AIRCRAFT 


14.1 INTRODUCTION 

14 . 1.1 The Aircraft Interception 

Problem 

The interception of enemy aircraft is one of the 
major problems of modern warfare. From the most 
general point of view this subject includes the entire 
radar warning system, all intelligence operations for 
obtaining information on enemy air action, the stra- 
tegic bombing of enemy aircraft factories, the anti- 
aircraft artillery commands, barrage balloons, the 
fighter commands, and all armament for defense of 
bombers against enemy fighters. This section dis- 
cusses only interception by fighter planes which use 
their own radar equipment at some stage in closing 
with the enemy planes. Although this aircraft inter- 
ception [AI] equipment may be used under any con- 
ditions of poor visibility, it has been used primarily 
in night fighting. 

Two main tactical methods are employed in bring- 
ing fighters into contact with enemy planes. These 
are ground controlled interception [GCI] and inde- 
pendent interception. In the GCI system, enemy 
planes are located by a complex surveillance network 
of which the major elements are ground radar sys- 
tems. The fighter is sent up and establishes radio con- 
tact with a control center which guides him to within 
a few miles of the enemy. Final closing is made with 
the aid of the fighter plane’s radar. Ship-controlled 
interception [SCI] in which carrier-based planes are 
guided into radar contact with enemy planes can be 
thought of as a special case of GCI control. 

On the other hand, there is the independently oper- 
ated plane which operates outside of GCI control. 
The principal use of this system is for “intruder” 
missions over enemy territory, for defense of a region 
in which a surveillance network is not yet established, 
or for defense against attackers flying so low or so 
fast that GCI is impractical. 

Aircraft interception sets differ primarily from 
aircraft to surface vessel [ASV] sets in the requirement 
that relative elevation information must be supplied. 
If a scanning system is used, this means the addition 
of an added degree of freedom to the scanner motion 
to give elevation information as well as azimuth in- 
formation. Thus, the AI scanner problem is more 


difficult than the ASV scanner problem (see Chap- 
ter 15). Considerable difficulties are also caused by 
sea and ground return obscuring the relatively weak 
aircraft signals. 

A major problem in all types of AI work is the 
provision of adequate identification of friend or foe 
[IFF]. This is necessary both for the surveillance 
radar and for the interceptor plane. Especially in the 
case of blind firing by radar, it is desirable for the 
interceptor pilot to feel confident of the identification 
of his target as an enemy. An efficient IFF system 
can also save much effort which might be wasted in 
the interception of aircraft that turn out to be 
friendly. 

Another important general problem is that of pro- 
viding, in the design of both ground and airborne 
radars, a maximum effectiveness against counter- 
measures which the enemy may use, such as “win- 
dow” or “carpet.” 

American radar sets designed primarily for AI use 
are: SCR-540 series (British Mark IV); SCR-520; 
SCR-720 series; AN/APG-1 and -2; AIA; AN/APS-6 
and -6 A; AN/APS-19 (in development at the close 
of World War II); and AN/APS-21 (in planning 
stage at the close of World War II). The AN/APG-3 
and AN/APG-16 systems, designed for control of 
firing from bomber turrets, have radar search facil- 
ities and might be adapted to AI work. 

14.1.2 Future Trends in AI 

The future of air warfare is uncertain, but two facts 
stand out clearly. One is the great increase in speed 
of the attacking enemy planes; the other is the ex- 
pected increased use of weapons similar to the Ger- 
man V-2 rocket. Improved AI tactics can probably 
cope with increased speeds in the range from 500 to 
800 mph. However, AI as now understood cannot be 
expected to function successfully against weapons of 
the V-2 type. 

The implications of higher speeds are many. 
Greater distances will be involved in an interception 
operation, and therefore greater radar range will be 
required. The necessity for detecting evasive action 
at the earliest possible moment means that higher 
scanning speeds must be used. The more pointed 
aerodynamic shapes of high-speed aircraft will make 
installation of AI scanners more difficult. 


176 


THE INTERCEPTION OF ENEMY AIRCRAFT 


It is to be expected that increased use will be made 
of rockets by fighters. This means that the gunlaying 
radar associated with the AI system should have the 
necessary computers for rocket firing. Use of infrared 
techniques, especially in the closing stages, may be a 
very valuable adjunct to the radar. It may thus be 
possible, for example, to detect at an earlier stage 
the beginning of evasive action as indicated by bank- 
ing of the hostile plane. 

The moving target indication [MTI] technique now 
being developed for ground sets and for airborne de- 
tection of moving vehicles (Chapter 23) may provide 
an answer to the ground and sea clutter problem. At 
present such equipment costs too much in weight, 
but this objection does not appear to be insuperable. 
Considerable development will be required before the 
value of such techniques for AI will be established. 

14.2 CONTROLLED INTERCEPTION 

14.2.1 The Surveillance Network 

A detailed discussion of the surveillance network 
for air defense cannot be given here, since the dis- 
cussion is primarily concerned with airborne radar. 
The problem is essentially one of creating a vast or- 
ganization for gathering information on enemy air- 
craft from radar, spotters, and intelligence services, 
and distribution of the information to antiaircraft 
and fighter commands. The fighter-direction centers 
direct the friendly fighters to a position where radar 
contact with the enemy is obtained. From then on 
the fighter is on his own, and it is with this aspect of 
the interception that we are primarily concerned. A 
discussion of AI tactics is given in Chapter 16. 

For SCI work the available information is more 
limited, and hence the problem of defending a task 
force from enemy air attack is one of great difficulty. 
All airborne search radars of the Cadillac type, scout- 
ing planes, outlying fleet units, and the ship-based 
radars must supply information to the control centers 
which guide the defending carrier-based fighters. 

14.2.2 The Radar Set for the 

Controlled Fighter 

In World War II, controlled fighters have been 
both single- and multi-place planes. The Navy has 
favored single-place planes for carrier-based work, 
whereas in the defense of England the larger two- 
place planes were used. 

The trend toward interceptor planes to give maxi- 


mum speed will probably require a small fixed-gun 
plane whose one-man crew is both pilot and radar 
operator. The radar must therefore be extremely 
light, compact, and simple in operation. Since the GCI 
director guides the fighter to within a few miles of the 
enemy, the requirements on range are not so great 
as for an independent plane which has to seek its 
quarry. Navigation features and defensive radars 
such as tail warning are not essential, although they 
are desirable to relieve congestion and responsibility 
at the control center, and to allow pursuit of very 
fast or low-flying attackers. Chapter 15 presents a 
more detailed discussion of AI requirements and de- 
sign problems. 

In the class of sets suitable for controlled inter- 
ception in single-place planes are the following sys- 
tems: SCR-540 ; AIA; AN/APS-6; AN/APS-19. The 
AN/APG-3 and AN/APG-16 sets could probably be 
adapted to this use. Systems for controlled intercep- 
tion in two-place planes are the same as those listed 
in Section 14.3.2 for independent interceptors. 

14.3 INDEPENDENT INTERCEPTION 

14.3.1 The Tactical Problem 

There is a definite need for interceptor planes 
which can go out on their own to seek out and shoot 
down enemy planes. Raids can be broken up over 
enemy territory while enemy bombers are climbing 
to altitude or getting into formation. Escort of 
friendly bombers over hostile territory at night is an- 
other function of this type of plane, as is air defense 
of territory in which a surveillance system for GCI 
control is not yet established. 

14.3.2 The Radar for the Inde- 

pendent Interceptor 

The best aircraft for intruder missions is a large 
fighter capable of relatively long-range missions. In 
general, there will be a separate radar operator as 
well as the pilot, comprising a two-man crew. 

The radar installation must provide navigation 
and tail-warning functions as well as search, inter- 
ception, and gunlaying. Since search is a major func- 
tion, the radar must provide good range performance 
and wide angular coverage (at least 180 degrees for- 
ward). This means, of course, more power and 
greater mechanical complexity than in radars for 
single-place planes. Since there is a separate radar 


RESTRICTED "* 


GUNLAYING PROBLEMS ASSOCIATED WITH AI RADAR 


177 


operator, greater complexity of data presentation is 
allowable. 

The radar should provide both PPI presentation 
of the ground for navigational use and complete 
beacon facilities. The defensive tail-warning radar 
should be a separate set but should have its controls 
and display well integrated with the interception 
radar. 

AI radar systems suitable for this type of work are 
the SCR-720, AN/APS-21, and to a limited extent 
the AN/APG-1 and -2 (which have inadequate 
search facilities). 

A type of intruder mission in which the AI equi p- 
ment can be greatly simplified is that in which the 
intruder picks out a particular enemy airfield and at- 
tempts to shoot down enemy planes while they are 
landing or taking off. For this purpose a range of 
only 1 or 2 miles is required, and there is little need 
for elevation information. Hence radars of the ASV 
type, such as the AN/APS-4, have been used for this 
work. 

14.4 GUNLAYING PROBLEMS ASSOCI- 
ATED WITH AI RADAR 

The object of every interception is to shoot down 
the enemy plane. Hence, means must be provided for 
aiming the fighter’s guns, whether fixed or in a mov- 
able turret. Most AI interceptions in this war have 


been radar guided into visual contact, with the actual 
shooting achieved visually. Of the AI systems which 
saw service in World War II, some (SCR-540, -520, 
-720) have provided no radar gunlaying features; 
others (AIA, AN/APS-6) have noncomputing radar 
fire control. The fact that radar gunlaying has been 
little used in World War II does not mean that it will 
be unnecessary in future designs, but that it must be 
better and have more highly trained operators. 

Part IV of this book is devoted to airborne fire- 
control systems, and the detailed discussion there 
covers most of the points of interest in AI work. 
Chapter 16 includes a discussion of the AI tactics 
which are used to bring the fighter into the most 
effective firing position. 

For single-place controlled fighters, it will prob- 
ably be adequate to have the radar scanner and pres- 
entation switched from search to gunlaying when the 
target is within range. Automatic tracking by means 
of a conical scan would then feed the necessary data 
to a spot-error indicator with range wings (G scope). 

For the independent plane with separate radar 
operator, there are strong arguments for having a 
separate conical scanner for the gunlaying function. 
Thus, while the pilot is making the final approach, 
the radar operator can continue his search function. 
This would be particularly useful in the event that 
the automatic following were thrown off by evasive 
action. 



Chapter 15 

AI EQUIPMENT 


15.1 DESIGN CONSIDERATIONS 

15.1.1 Introduction 

This chapter discusses the aircraft interception [AI] 
radar equipment carried in the interceptor aircraft. 
The design parameters influencing the performance 
of AI equipment, and the specific problems associated 
with AI equipment, together with examples of solu- 
tions of these problems as they have been met in 
practice, are presented. The emphasis will be on the 
components that are peculiar to the tactical function 
of AI radar equipment; consequently, the scan and 
the presentation are discussed in some detail. 

The gunlaying feature of AI is omitted from sev- 
eral of the sets and is rarely used even in the sets that 
have it. Few pilots are willing to shoot blindly at a 
target that may prove friendly. Lack of faith in 
equipment for identification of friend or foe [IFF] 
has prevented the wide use of the blind firing facil- 
ity in combat; the preference is to fire visually after 
visual identification. Accordingly, airborne radar 
fire-control facilities will not be discussed in this 
connection. (See however Part IV.) 

15.1.2 Beamwidth 

The width of an antenna beam as measured in the 
horizontal or azimuth plane is given in degrees by 
the approximate formula 


where X and D are, respectively, the wavelength of 
the radiation and the horizontal dimension of the 
reflector. The same equation shows how the beam- 
width as measured in elevation is influenced by the 
vertical dimension of the reflector. Narrow beams are 
obtained by the use of a short wavelength or a large 
antenna or both. The above formula agrees fairly 
well with measurements on most antennas. 

In modern AI sets the beamwidth is 5 or 10 de- 
grees. It is obvious that narrow beams are desirable 
as affording better accuracy and resolution. The 
value of narrow beams in the reduction of ground and 
sea clutter is discussed in Section 15.1.6. A K-band 
beam as narrow as 1 degree might make possible a 
crude display of the actual outline of the target air- 
plane, so that the interceptor pilot could, by observ- 
ing its banking, react sooner to its evasive tactics. 


The AI scanning problem is, however, so aggravated 
by such a narrow beam that its use has not been 
seriously planned. 


15.1.3 Range Performance 21a 

Range Equation and Antenna Gain 
The maximum range # max at which a radar can de- 
tect a target depends on several quantities, including 
the wavelength, antenna gain, peak power P transmit- 
ted, the signal power p min in the weakest discernible 
echo, and the effective cross section a of the target. 
The received power is given by 

PGWa 

P ~ (4 r) 3 R 4 ^ 

where G is the gain of the antenna, and R is the range 
to the target (see Section 7.2.2). 

It follows that 



In the above equations the quantities G and p min 
depend on definite attributes of the radar system. 
The antenna gain G measures the directivity of the 
antenna; it is defined as the ratio of the energy 
density at the maximum of the beam to that from an 
equal source radiating isotropically. The gain of a 
pencil-beam antenna of area A is given by the 
formula 


G = 


ItAF 

X 2 


( 4 ) 


where F is a constant depending on the antenna de- 
sign, and is approximately 0.5 for AI antennas. 


Scanning Loss 

The sensitivity of the radar as described by the 
power p min in the minimum discernible signal de- 
pends on many factors, and a complete discussion is 
beyond the scope of the present book. Only the effect 
of scanning loss, recurrence frequency, and pulse du- 
ration will be considered. In discussing scanning loss 
we define p ' min as the minimum discernible signal 
when the beam is steadily trained (searchlighting) on 
the target. Then 

Pmin = fp min (5) 

where / is designated the scanning loss factor. A 


DESIGN CONSIDERATIONS 


179 


rough rule for approximating the scanning loss is 



where n is the fraction of transmitted pulses of radi- 
ation which fall on the target. This holds approxi- 
mately for scans which are completed in a time of 
about 6 seconds or less, as is the case with AI scans ; 
it finds a certain justification both in the theory of 
measurements and in actual observation under con- 
trolled conditions. Six seconds represents a combined 
integration time of the observer and the persistence 
or afterglow of the cathode-ray screen. 

For uniform scanning rates 

Solid angle £2i of beam 

71 = ) 

Solid angle £2 scanned 

and therefore 



The foregoing discussion of scanning loss has been 
based on the tacit assumption that the signals appear 
at the same place on the screen in successive repe- 
titions of the scan. If, as is generally the case in AI, 
this assumption fails to hold, a further loss in sensi- 
tivity of the radar results, involving perhaps a factor 
of 2 or more in minimum discernible signal. For pur- 
poses of comparing the performance of AI sets, how- 
ever, this additional loss may be neglected as being 
of the same order of magnitude for all systems. 

The problem of scanning loss has been treated by 
E. M. Purcell. 21a He advances the point of view that, 
since both the target and the screen are scanned and 
pulsed repeatedly, it is profitable to consider not the 
scanning loss referred to the searchlighting condition, 
but rather a storage gain referred to the case of a 
radar which is trained on the target while emitting 
an isolated pulse. 

If the scanning is nonuniform, the angular velocity 
co of the beam must be taken into account, and in 
general co is a function of two angular variables. Each 
scanning of the field is accomplished by traversing 
several lines of scan. If the lines are so close together 
that the beam falls on a given target while traversing 
several successive lines, the scanning loss is miti- 
gated. An overlap factor k is therefore defined as the 
ratio of the beamwidth to the angular shift of the 
beam between lines. This is a function of the two 
angular variables. Let t F denote the frame time (time 
for completely scanning the field once), v r the pulse 
recurrence frequency and 9 the beamwidth. Then 


for any beam direction, the number of pulses on the 
target in one frame is v r Ok/ co, the total number of 
pulses in one frame is v r t F and we find, using equa- 
tion (6), 

/ = ( t F a/Ok )K (8) 

Effect of Pulse Duration, Bandwidth, and Re- 
currence Frequency 

The pulse duration of the transmitted energy has 
an indirect, though important, effect on the value of 
p'min. The pulse duration r (microseconds) fixes the 
optimum bandwidth of the receiver (megacycles) at 
about 1.2/ r. Thus, if the pulse duration is decreased 
for the sake of improved resolution of targets in 
range, the bandwidth must be increased. This gives 
a higher noise level and results in a decrease in signal 
discernibility. The resulting relation between the 
pulse duration and the minimum signal discernible 
while searchlighting is 

P'min ~ * (9) 

T 

The recurrence frequency also affects the value of 
p' min. It is found [see equation (5)] that 



Consolidation of Design-Parameter Effects 
On substituting equations (4), (5), (9), and (10) 
into (3), we find the following dependence of the 
range performance of an AI radar upon design param- 
eters : 

(. 1 ) 

The value of / is calculated from equation (7) or (8). 
By equation (11) one may compare the range per- 
formance of AI radars and so arrive at an optimum 
design of new sets. 

15.1.4 Angular Coverage and 
Frame Speed 

The AI radars that have been in production have 
widely differing fields of view. It is important that 
the angular coverage for the search phase of AI 
should be ±90 degrees in azimuth (Section 16.2.2) 
and 20 degrees total in elevation, with the center 
adjustable by the operator from —10 to +10 de- 
grees. 13 These requirements are for an interceptor on 
a free-lance mission, i.e., not under ground control. 
For controlled AI search the azimuth coverage need 


RESTRICTED 


180 


AI EQUIPMENT 


not be so great, and ± 60 degrees is considered suffi- 
cient. In either case a frame time of 3 seconds is 
short enough. 

The final interception phase of AI, after the enemy 
has begun evasive action, requires less range but 
wider angular coverage. The ±90 degrees figure for 
the azimuth coverage is still necessary, and the ele- 
vation coverage should be increased to at least a 
total of 30 degrees with the center adjustable from 
— 15 to +45 degrees. The tactical situation in air 
combat changes so rapidly that the frame time should 
be reduced to 1 second. 

The angular coverage and frame speed of specific 
AI radars are discussed in Section 15.2.1. 

15.1.5 Navigation by AI Radar 

It often happens that a radar is useful not only in 
its principal function but also in other functions as 
well. This is true of AI, which has proved of use in 
navigating en route to an intruder operation over 
enemy territory, and which is very valuable in the 
return flight to the airplane carrier or base of opera- 
tions. This navigational facility is usually inferior to 
that of radars designed primarily for navigation, 
because the AI scans are primarily adapted for 
searching space rather than for detecting ground ob- 
jects, and because the pencil beam from most AI 
antennas is less efficient in scanning the ground than 
is the fan beam of airborne navigational antennas. 

In the detection of land masses or ships, the range 
performance is better than in the detection of indi- 
vidual aircraft. Therefore, particularly with high 
performance sets now under design, distant signals 
may appear on the second sweep. Various means 
have been proposed for eliminating this difficulty. 
One method is to vary the pulse recurrence frequency 
so that the unwanted echoes do not appear at the 
same place on the type B or the plan position indi- 
cator [PPI] display on successive sweeps. 

The ability to interrogate beacons is of great value 
to an interceptor pilot returning to his base, and this 
ability is specified in practically all AI radars. 

15.1.6 Ground and Sea Clutter 

In AI operations it is observed that echoes from 
objects on the ground or from waves on the sea often 
seriously mask the echoes from the target airplane. 
In a certain sense, this surface clutter is a conven- 
ience in that by observing it on the screen the oper- 
ator can estimate his altitude and the attitude of his 


airplane. Nevertheless, much effort has been ex- 
pended in suppressing this generally bothersome 
effect. 

In addition to the usual widely spread surface 
echoes, an altitude signal generally appears. This is 
an echo from the surface vertically below the air- 
plane and is observed despite the fact that the beam 
is never directed vertically downward. It is caused by 
diffuse general radiation from the antenna and by 
downward reflections of the main beam by the 
radome. It may mask the signal from an airplane 
which is at a distance equal to the altitude of the 
interceptor. 

Surface clutter may be reduced in several ways. 
One of the two most direct methods is to use short 
pulses; the other is to use a narrow beam. The effi- 
cacy of these methods will be understood from the 
fact that the signal from the target is in competition 
with the signal from an area on the surface. 

The signal strength p* from the surface is propor- 
tional to G 2 and to <r*, the radar cross section of the 
surface area in question. Since the reflections from 
such things as trees and waves combine incoherently, 
this cross section is proportional to the area on the 
ground illuminated by the pulse which simultane- 
ously strikes the target aircraft. This area is pro- 
portional to RQt at ranges large compared with the 
altitude. 

Using equation (2) we find that 


p* 


PG 2 \ 2 RQt 



R 4 


( 12 ) 


In the competition between p [see equations (1) and 
(2)] and p* it is desirable to minimize 

p* RQt R\t 

— ~ ~ — • (13) 

p a crD 

There is a benefit from short pulses and from beams 
which are sharp in azimuth. For example, if the 
width of the reflector is doubled, the surface clutter 
is doubled but the signal strength from the target is 
quadrupled; the net result is advantageous. Clutter 
is not diminished in relation to the desired signals by 
increasing the vertical aperture of the antenna. 

We have seen that range performance is improved 
by long pulses, but that surface clutter is reduced by 
short pulses. Also range performance is improved by 
a narrow beam, but the problems of scanning and 
scanner design are made more difficult by narrow 
beams. These factors must be kept in mind in choos- 
ing the optimum pulse duration and beamwidth. 


'H 


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DESIGN CONSIDERATIONS 


181 


Reduction of the altitude signal requires a reduc- 
tion of the vertically downward radiation. With this 
in mind, the antenna should be designed to have side 
lobes as low as possible; 25 db below the beam in- 
tensity is a fairly good figure. A sheet-metal lining in 
the lower part of the radome has been found to help 
in SCR-720 installations by blocking the downward 
side lobes. If the scanner installation is poorly de- 
signed, the altitude signal may persist despite the 
choice of an antenna with low side lobes; this may 
happen if the radome reflects part of the beam 
energy downward or if a propeller or other part of the 
airplane is at any time illuminated by the scanning 
beam. 

Surface clutter in the AN /APS-6 system is partly 
eliminated by cutting off the receiver during the 
time the scanner points below a certain elevation 
angle which can be controlled manually by the oper- 
ator. This, of course, creates a blind volume below 
and in front of the airplane. A somewhat similar 
method proposed for the AN/APS-19 is to gate the 
receiver so that, when the beam is inclined down- 
ward, amplification takes place only until the time 
corresponding to the slant range to the ground. This 
means that the elevation angle and altitude informa- 
tion must be fed to a computer which can compute 
the product of the altitude by the cosecant of the 
depression angle of the beam, and that circuits must 
be provided to produce the computed gate length. 

Various circuits have been developed to prevent 
saturation in the system resulting from clouds, sea, 
or land. A number of these circuits have been treated 
in detail as applied to the Cadillac system. 17 They 
will merely be listed here: (1) sensitivity time control 
[STC], (2) instantaneous automatic gain control 
[IAGC], (3) fast time constant [FTC] coupling be- 
tween detector and video amplifier, (4) detector 
balanced bias [DBB] circuit. 

As mentioned in Section 14.1, it is now possible to 
incorporate an airborne moving target indicator 
[AMTI] into an AI radar, enabling a display of 
objects which are moving in relation to the surface 
or to the interceptor while suppressing the signals 
from fixed surface objects. This technique is advan- 
tageous only on targets that are at a slant range ex- 
ceeding the altitude of the interceptor. 

15.1.7 AI Scans 

The beam of the interceptor radar must be made 
to search a specified solid angle. This search may be 


made with a fan beam brushing over the field once 
every second or two, but such a scan would give data 
on only one of the two angular coordinates of the 
target. Such a scan would have a more favorable 
scanning loss value [see equation (7)] and a less 
favorable antenna area and gain than would a pencil- 
beam scan of several lines rapidly traversed. The 
range performance of the latter scan is better; equa- 
tion (11) shows that the range advantage in this re- 
spect is a factor equaling in order of magnitude the 
% root of the number of lines in the complex scan. 
(Substitution of a pencil beam for a fan beam would 
multiply / by the square root of the number N of 
lines in the scan and would allow the gain or A to be 
increased by the factor N. The net alteration in 
maximum range is as stated.) The complex scans 8 
are therefore preferred, both because they enable 
more definite locating of a target and because they 
allow earlier detection. 


1 o 1b 

2 



Figure 1. Types of AI scan. 

Six complex scans have been considered for the AI 
function. These are shown schematically in Figure 1 
and may be classified as: 

1. Helical scan. 

a. Single reflector. 

b. Two reflectors back to back. 

2. Spiral scan. 

3. Wigwag scan. 

a. Rapid azimuth scan with slow elevation 

scan. 

b. Rapid elevation scan with slow azimuth 

scan. 

4. Palmer scan (rapid conical scan with slow 
azimuth scan). 

Of these scans, 2, 3b, and 4 involve rapid vertical 
motion of the beam. This is a bad feature as regards 
the B or PPI presentation, since it means that the 



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182 


AI EQUIPMENT 


same part of the screen where the target is observed 
is immediately and repeatedly filled with surface 
clutter. The much slower elevation motion of scans 
la, lb, and 3a is free from this objection, and these 
scans are preferred. 

The back-to-back helical scanner lb is undesirable 
on the ground of excessive size, since the reflectors 
must be simultaneously tilted up (or down). It is 
easy to see that at the 60-degree elevation angle, 
which is the maximum required, the swept diameter 
of the antennas would have to be over two-thirds 
more than the reflector diameter, which is the swept 
diameter required for a single scanning reflector. 

On the basis of the above discussion the choice of 
scan is narrowed to helical with a single reflector, or 
a horizontal wigwag scan. Further discussion of 
scans will be found below in the descriptions of spe- 
cific AI systems. 

15.1.8 Stabilization of AI Scanners 

It has been thought that gyro-stabilization should 
be provided for AI scanners. Such a feature might be 
useful in the search phase, since then any banking of 
the interceptor would not cause a corresponding 
tilting of the rectangular field of scan. More impor- 
tant would be the benefit of stabilization during the 
interception phase when the target may be under- 
taking evasive action. Suppose that the interceptor 
radar, not stabilized, has a type C (azimuth-eleva- 
tion) display and that, as is frequently true, the 
interceptor is pursuing behind and below the target. 
If now the target turns right, the interceptor pilot 
will turn right. This requires a steep right bank, 
which will put the azimuth of the target (referred to 
the tilted “vertical” axis of the interceptor scanner) 
to the left. The pilot will accordingly bank to the 
left, whereas he should be continuing his right bank. 
In this hypothetical situation, as in many others, 
there is confusion of the azimuth coordinate as re- 
ferred to the banking airplane with the azimuth re- 
ferred to a truly vertical axis. The elevation angle is 
subject to a similar confusion. This condition, possi- 
ble only with an unstabilized radar, may be avoided 
by stabilizing either the scanner or the displayed 
data. Data stabilization in regard to roll would prob- 
ably not be difficult or costly to apply to spirally 
scanning AI radars. The choice of method of stabi- 
lization will not be further discussed. Although in 
the foregoing the display was assumed to be a C 
scope, the argument is valid for B or PPI scopes as 


well. In the AN/APG-1 set (see Chapter 18) the 
radar artificial horizon reduces the need for stabi- 
lization. 

15.1.9 AI Presentation 

The indicator display presented by an AI radar 20a 
is chosen first and foremost according as the inter- 
ceptor does or does not carry a radar operator in 
addition to the pilot. 

For one-place airplanes the double dot modifica- 
tion of a B scope has almost universally been used. 
The image of a single target is double, with the left- 
hand image indicating the range of the target and a 
theoretical centerline between the two dots indicat- 
ing the azimuth. The right-hand image is above or 
below the other at an angle which varies as the target 
is above or below the axis of the interceptor. The 
cramped quarters of the cockpit have limited the 
diameter of the screen to 3 in., although a larger 
screen would be easier to read. As the interceptor 
closes on a target, the pilot changes successively from 
long-range to progressively shorter-range sweeps. 
However, it would seem that the changes provided 
should not be greater than a factor of about 2.5. 
Pilots have reported losing a target in changing from 
a 5-mile to a 1-mile sweep. 

All AI sets designed for pilot operation provide 
blind-firing facility, consisting of a conical scan indi- 
cated on a G scope. This scope presents a single spot 
which qualitatively indicates the direction and mag- 
nitude of the error in aiming the axis of the conical 
scan. The target so indicated is the nearest one 
scanned, and if the two nearest targets are close to- 
gether and at the same range the spot indicates, as 
it were, their center of gravity. Qualitatively the 
range is indicated on a G scope by wings electroni- 
cally painted on either side of the spot, which grow 
as the interceptor closes. 

AI for two-place airplanes may have not only a 
B scope for the operator but also a C scope, used 
after a target has been detected on the B scope. It is 
necessary for the operator to keep a range gate ad- 
justed on the target in order to reduce the noise on 
the C scope. The position of the gate is controlled by 
a knob, allowing the operator to track the target in 
range; the tracking is made possible by range data 
read from the B scope. The pilot has a single scope 
which can be made type B or C at will. There is also 
provided for the pilot a range meter which simply 
repeats the position of the gate-control knob. In some 


' ^ RESTRICTED 


AI SYSTEMS 


183 


cases the operator also needs a G scope for gun aim- 
ing. 

15.2 AI SYSTEMS 

15.2.1 American AI Radars 

There have been designed in this country at least 
nine AI radars or radars adaptable to AI. These are 
SCR-520, SCR-540, SCR-720, AIA, AN/APS-4, 
AN/APS-6A, AN/APS-6, AN/APG-1, and AN/- 
APG-2; all except the last two have been produced 
in quantity. In addition, three sets (AN/APS-19, 
AN/APG-3, and AN/APG-1 6, the last two of which 
are for bomber turret fire control) are in the stage of 
development and test, and one (AN/APS-21) is being 
planned. 

The SCR-540 system, adapted from the British 
Mark IV AI radar, was manufactured by Western 
Electric Co. It has five fixed dipole antennas which 
do not scan, and operates at 193 me. One of the an- 
tennas transmits about 800 pulses per second, of 
1.3 jusec duration, diffusely in the forward direction, 


and the other four antennas receive the signals. The 
receiving antennas are directionally sensitive and are 
installed not quite parallel. They are connected in 
turn to the receiver and thus enable lobe switching. 
The display is a 3-in. double A scope with the base 
lines back to back (L scope) providing rough azimuth 
information on the target, as in the ASB radar. A 
second L scope, using the upper and lower antennas, 
provides elevation data. 

Four of the sets mentioned were intended not only 
for interceptors but also for bombers. They are 
AN/APG-1, AN/APG-2, AN/APG-3, and AN/- 
APG-16. AN/APG-1 and AN/APG-2, manufactured 
by Western Electric and General Electric, are similar 
in many respects. The former is described in Table 1. 
All four of these sets are discussed in Chapter 18. 
These systems are especially adaptable to the gun- 
laying function of aircraft interception. In this re- 
spect, the small size of the AN/APG-3 (General 
Electric) and AN/APG-16 (Sperry) gunlaying sets 
for bombers makes them especially appropriate. 

The SCR-520 radar is of historical interest only as 


Table 1 . Characteristics of typical AI equipments. 



SCR 720 

AN/APG-1 

AN/APS-6 

AN/APS-19 

AN/APS-21 a 

Manufacturer 

Western Electric 

Western Electric 

Westinghouse 

Sperry 

Westinghouse 

Prod, date 

1943 

1943 

1944 

1946 

? 

Service 

Army 

Army 

Navy 

Navy 

Navy 

Frequency 

3,000 me 

3,000 me 

9,375 me 

9,375 me 

9,375 me 

Search scan 

Helical 

Palmer 

Spiral 

Spiral 1 * 

? 

Center (elev.) 

Adjustable 

2*° 

Ahead 

Ahead 

- 15° to + 45° 

Field (elev.) 

- 30° to + 50° 

30° 

120° 

130° or 30° 

30° 

Field (az.) 

± 90° 

165° 

120° 

130° or 30° 

180° 

Fast motion 

6 c 

4,000 rpm 

20 c 

20 c 

? 

Frame time (sec) 

Typically 1 

8 

1 

1 or I 

1 

Gunlaying scan 

None 

Conical 

Conical 

None 

Separate radar 

Center 


- 10° to + 68° 

Ahead 


? 

rpm 


4,000 

1,200 


? 

Antenna Diameter (in.) 

29 

27 

m 

18 

30 

Beamwidth 

10° 

10° 

5° 

5° (esc 2 ) 

3.4° 

Gain 

340 

310 

1,100 

1,100 

2,400 ? 

Peak power 

100-150 kw 

125 kw 

40 kw 

40 kw 

250 kw 

Radar pulses per sec 

1,500 

1,600 

1,000, 2,000 

2,000, 4,000 

1,600 

Radar pulse duration (*isec) 

3 

4 

! 

1, h 

i i 

2) 4 

3 1 

4 , 1 

Beacon pulses per sec 

375 

400 

500 

500 

400 

Beacon pulse duration (jusec) 

21 

21 

2 

2 

2 

Operator’s indication 

5 in. B scope 

5 in. C scope 

5 in. B or C scope 

Pilot operated 

Pilot operated 

7 in. PPI, off ctr 
5 in. C scope 

Pilot’s AI indication 

3 in. B or C scope 

3 in. G scope, 

3 in. B 0 d or 

3 in. B, B 0 d or 

5 in. PPI 


and range meter 

range meter 0 

G scope 

V e scope 

3 in. C scope 

Radar sweeps 

26,000 ft, 10, 20, 
100 mi 

5,000, 20,000 yd, 
100 mi 

1, 5, 25, 65 mi 

1,500 yd, 2, 8, 20, 
50, 100 mi 

5 to 80 mi 

Beacon sweep 

100 mi 

100 mi 

100 mi 

150 mi 

160 mi 

Range on fighter 

5 to 8 mi 

5 to 8 mi 

3 to 4 mi 

4 to 7 mi 

15 to 20 mi 

Gunlaying indication 

None 

Turret gun AGL 

Fixed gun G scope None 

Separate radar 

Weight (lb) 

368 

574 

242 

180 (inc bomb 40) 600 lb f 

Power requirement 

2.7 kw 

3.6 kw 

1.6 kw 

1.5 kw 

? 


a Proposed system; question marks indicate undecided factors. d Double dot modification. 

b Also has sector scan. e As in AN/APG-13B ( Vulture ); see Fig. 21, Chapter 20. 

c Also overtaking meter and radar artificial horizon. f Including the associated GL and tail warning radars. 


184 


AI EQUIPMENT 



Figure 2. AN/APS-6 system components. 


the direct ancestor of the lighter and more powerful 
SCR-720. Since these sets are rather similar, only the 
latter needs to be described. Its main attributes are 
displayed in Table 1. 

In the AN/APS-4 radar manufactured by Western 
Electric the Navy has a widely used lightweight 
navigational radar with a sector scan indicated on a 
B scope. It was expected that optional automatic 
variation of the elevation angle, resulting in a four- 
line wigwag scan, would enable the set to serve effi- 
ciently in an AI capacity. This expectation has been 
only partly confirmed, although the set has been ex- 
tensively used for AI missions against specific enemy 
airfields. 

The AIA radar (manufactured by Sperry) for 
Navy night fighters was an early X band AI set af- 
fording gunlaying facility. It has been superseded 
by the somewhat similar AN/APS-6 A and AN/APS- 
6 sets. The obsolescence of AIA was hastened by the 
undesirable long waveguide transmission line be- 
tween the magnetron, packaged with the modulator 
in the fuselage, and the scanner mounted in a nacelle 
on the leading edge of the right wing. Troubles with 
installation, maintenance, attenuation, and the long- 
line effect were serious. 

AN/APS-6 A and AN/APS-6 avoid these difficul- 


ties by mounting the magnetron in the r-f head in 
the nacelle close behind the scanner. It is true that 
the pulse cable leading from the modulator in the 
fuselage to the r-f head is long, but the attendant dis- 
advantages are slight. The only important difference 
between AN/APS-6A and AN/APS-6 is that the 
former was an interim set, which used the r-f head 
designed for AN/APS-3 until heads could be de- 
veloped especially for AN/APS-6. Figure 2 is a pho- 
tograph of the AN/APS-6 system. 

During the latter part of World War II, a feeling 
arose that AN/APS-6 was too heavy for single-place 
airplanes and of too low a performance for two-place 
airplanes, that SCR-720 allowed too much surface 
clutter because of its great beamwidth, and that 
both sets were in many ways technically obsolescent. 
Therefore, two AI developments were initiated by 
the Navy, one toward an improved and lighter sys- 
tem for carrier-based single-place night fighters 
(AN/APS-19) and the other toward a high perform- 
ance set for interceptors large enough to carry a 
radar operator (AN/APS-21). Completely different 
designs have resulted. 

AN/ APS- 19 is designed for light weight and there- 
fore only 4- to 7-mile range performance on such tar- 
gets as enemy torpedo bombers. Since it is not in- 


AI SYSTEMS 


185 


tended for long-range search, its targets are perhaps 
almost as likely to be found above or below the axis 
of the airplane as to the right or left. Therefore, a 
130-degree spiral scan is used. Since in AN/APS-6 
the conical scan and the type G display give a field 
of coverage only about 15 degrees in diameter, the 
enemy has often eluded a pursuer by violent evasive 
tactics. Therefore, conical scan is not used in AN/- 
APS-19, but there is specified instead a supplemen- 
tary spiral scan of very fast frame time and 30-degree 
field. In order to improve surface search, land map- 
ping and beacon performance, a sector scan is also 
incorporated. During this scan the antenna feed is 
automatically altered so as to produce a fan beam 
instead of the usual pencil beam, thus enabling de- 
tection of near as well as distant objects on the 
surface. 

A possible set of requirements for the proposed 
high performance AI set, AN/APS-21, has been pre- 
pared by the Radiation Laboratory. 13 The data under 
this system in Table 1 are taken from this source. 
The desired range performance will be obtained by 
higher power and gain and by increased receiver 
sensitivity. The scanning problem is extremely diffi- 
cult in view of the short frame time, great angular 
coverage, and narrow beam. 

The two scans most seriously considered for AN/- 
APS-21 are, as mentioned in Section 15.1.7, the 
helical scan and the horizontal wigwag scan of Fig- 
ure 1. The choice of scan for AN/APS-21 is not easy. 
The scanning requirement for interception is more 
severe than for search, and will probably fix the 
choice of scanner type. Supposing the 30-degree ele- 
vation zone to be scanned by 12 horizontal lines at 

2.5 degrees spacing, a helical scanner would have to 
rotate at 720 rpm or a wigwag scanner would have 
to oscillate at 360 cycles per minute in order to 
achieve a 1-sec frame time. Assuming a pulse recur- 
rence frequency of 1,600 per sec and using equa- 
tion (7), we calculate the scanning loss factor to be 

30.5 for the helical scanner or 27.1 for the wigwag 
type. The latter has a very slight advantage. 

A wigwag scanner design 12 has been developed 
which is adaptable to the requirement of AN/APS- 
21. This uses an elastic restoring torque on the verti- 
cal wigwag axis of the antenna, of a magnitude so 
chosen that the antenna oscillates like the balance 
wheel of a watch at the desired frequency. The oscil- 
lation of this scanner is sustained by an electric 
motor; the amplitude may be set by the operator at 
will, enabling a mitigation of the scanning loss within 


a narrowed sector if desired. By clutching a flywheel 
to the motor shaft it could be made possible for the 
operator to have a slower scan rate at will. 

A seven-inch PPI presentation was recommended 
for the AN/APS-21. The most efficient use of the 
screen would require the center of the display to be 
offset downward to about one-third the radius of the 
screen; below this point the screen would be blank. 

15.2.2 AI Maintenance 

Introduction 

A more complete discussion of the problem of 
radar maintenance is given in Section 5.1. Although 
it was written for ASV radar systems, all the infor- 
mation included in it is pertinent and directly appli- 
cable to AI radar equipment. The design and instal- 
lation of AI systems, however, accentuate some of 
the aspects of the radar maintenance problem. Thus, 
for example, the mounting of the r-f head in an 
AN/APS-6 or AN/APS-6 A system close behind the 
scanner required that very careful consideration be 
given to the design of the directional coupler for 
these systems so that it could be suitably installed 
and its output made readily available for test pur- 
poses. Unfortunately the design, installation, and 
dispatch of AN/APS-6 and AN/APS-6 A systems to 
active theaters was accomplished prior to the design 
of a satisfactory directional coupler. It was necessary, 
therefore, to design a directional coupler for retro- 
active installation. 

Special AI Test Equipment 

Since the scanner and r-f head of an AN/APS-6 
system, for example, are mounted in the wing nacelle 
of a carrier-based night fighter and since the wings 
are folded back during any maintenance, it is neces- 
sary to install the test panel in the wing nacelle near 
the junction box, and in such a manner that all the 
test points are readily accessible. The test points to 
be included are discussed in Section 5.1.4. A control 
for a movable range mark should be provided on the 
test panel, if the test video contains such a range 
mark for echo box measurements. Although these 
remarks pertain primarily to a system of the AN/- 
APS-6 type installed in a Navy night fighter they can 
be easily generalized to include other types of AI 
systems and other types of installations. 

Three items of test equipment are rather special to 
some AI systems (although two of them, or suitable 
modifications, are used with other radar equipments) ; 


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186 


AI EQUIPMENT 


they are power absorption cone, boresighting equip- 
ment, and scanner balancer. 

The power absorption cone (or screen) is designed 
to absorb radiated energy from the antenna to pre- 
vent interference with nearby systems and reflections 
from nearby objects. This equipment is necessary 
during tests inside a hangar. 

Boresighting is the method of aligning the guns 
with the scanner, for those systems which have a 
gunlaying scan and indication, such as the AN/- 
APS-6, -6 A, and -19. The SCR-720 type of system 
has no gunlaying scan and indication ; consequently, 
no boresighting is done for the systems of this type. 
The two methods of accomplishing the alignment of 
the guns with the scanner are optical and radar. Up 
to the end of the hostilities of World War II the 
optical method was used with AN/APS-6 systems, 
since no suitable radar method had yet been de- 
veloped. Experience with the radar boresighting of 
other systems 14 - 15 indicated, however, that the ac- 
curacies to be attained would surpass those of the 
optical method. 

The following is a description of the optical method of 
boresighting the AN/APS-6 system. The aircraft is blocked 
up in a horizontal position on jacks at a standard position in 
front of the hangar wall on which are standard marks at which 
the guns are pointed. A boresighting telescope, of the diagonal 
vision type, is fastened to the yoke of the antenna by means of 
a right angle bracket. The telescope is pointed at the bore- 
sighting chart on the hangar wall, and is rotated about the 
axis of scan by manually rotating the yoke upon which it is 
mounted. The center of rotation, as determined by the tele- 
scope, is taken as the radar boresight point. The telescope 
need not be clamped accurately in position on the yoke, since 
misalignments cancel during the rotation process; it is clamped 
rigidly, however. The two sources of error in this method are : 
(1) radar reflections from the fuselage or other parts of the 
plane are neglected; and (2) the boresighting tool is mounted 
on the yoke which drives the dish rather than on the dish itself 
(the dish is allowed to point in any random position during 
boresighting). If, during conical scan, the dish does not de- 
scribe a circle about an axis coincident with that of the yoke, 
the boresight is in error, particularly if the dynamic orbit of 
the dish is not the same as the quasi-static orbit of the dish. 

Thus, a telescope of the diagonal vision type, a right angle 
bracket, and the boresighting chart are necessary items of test 
equipment for these systems. Since the optical method is used 
it is essential that the radome be easily and conveniently re- 
movable. 

A scanner balancer is a machine which measures 
and analyzes vibration in terms of amount and loca- 
tion of weight to be added to produce satisfactory 
operation. It is used for systems having spiral and 
conical scanners. The Gisholt scanner balancer ma- 
chine (GGJ-10AEH) was especially designed for 


systems such as the AIA, AN/APS-6, AN/APS-6 A 
and AN/APS-19; it has been used at major repair 
depots. 

15.2.3 The Installation of AI 
Equipment 

This discussion involves chiefly the installation of 
the scanner, which alone presents problems peculiar 
to AI. In single-engine night fighters the AI scanner 
is installed in a nacelle well outboard on the leading 
edge of the right wing. This location is chosen in order 
to avoid radar interference from the propeller and in 
order to allow unobstructed vision to the pilot as he 
circles for a landing. Installations of this sort that 
have been made in quantity are AIA, AN/APS-6A, 
and AN/APS-6 in F4U or F6F airplanes. The radar 
scanner is installed inside the nose of two-engine 
night fighters, such as the F7F and P-61. Almost the 
entire AN/APS-19 system may be mounted in a 
“bomb” under the right wing of single-engine air- 
planes; the nose of the bomb is the radome. AN/- 
APS-21 is intended for use in two-engine airplanes 
with the scanner located in the nose. 

Any scanner providing a gunlaying facility must be 
harmonized with the guns. The process of harmoni- 
zation or boresighting is simply the correct adjust- 
ment of the angular position of the scanner base, and 
means for this adjustment are provided in the scan- 
ner design. The tests for correct adjustment were 
discussed in Section 15.2.2. 

Problems concerning the r-f transmission line have 
been discussed above in connection with AIA. An 
additional difficulty arises from the shock mounting 
of the r-f head and the rigid mounting of the scanner. 
Their relative motion necessitates flexibility in the 
short length of transmission line that joins them. 

The radome of AI radar must be designed with un- 
usual care. First, it must be good aerodynamically 
since it is on a critical part of the exterior of a very 
high performance airplane. Second, it must be of the 
lowest possible reflectivity. A reason for this, pe- 
culiar to AI, is that reflections set up standing waves 
in the transmission line which, considering the rapid- 
ity of the scan, vary the frequency of the magnetron 
too rapidly for the automatic frequency control to 
follow; furthermore, reflections from the radome 
downward to the ground cause an increase in surface 
clutter and the altitude signal, which are especially 
unwelcome in AI. And third, the radome design must 
be such as to prevent the entrance of rain or spray, 


M 9 


AI SYSTEMS 


187 


whether the airplane is in flight or on the flight deck 
with its wings folded. Dew or a thin film of water 
may cause great reflection and attenuation. Im- 
proper mounting can allow several quarts of rain 
water to collect in a radome installed on a wing 
whose leading edge points downward when folded. 

It is imperative that ample tests be conducted to 
make sure that the antenna pattern is satisfactory. 
In the final stages of the testing and development, 
the scanner must be mounted in the proposed radome 
on a mockup of the intended airplane, and the time 
schedule must make provision for alterations to the 
antenna, the radome, the airplane or even to all 
three. 


An installation feature in the cockpit of an AI air- 
plane, which has been strongly advocated in England 
but thus far not used in this country, is windscreen 
projection. The cathode-ray tube is so mounted that 
the pilot readily sees its reflection in the flat window 
surface of the canopy before him. 

In the design of AI equipment, as in all airborne 
radar, it is important that the closest cooperation be 
encouraged by the interested service among the elec- 
tronic development agency, the equipment manu- 
facturer, and the aircraft manufacturer. The last 
mentioned has too often been left in ignorance of the 
space and other requirements of the radar, and a 
poor installation has resulted. 


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Chapter 16 

AIRCRAFT INTERCEPTION TACTICS 


16.1 EARLY TECHNIQUES 

16.1.1 Preradar Techniques 

The earliest techniques involving night fighters in 
the defense of territory against attacking enemy 
bombers utilized ground-based early warning sys- 
tems, searchlights, and day fighters operating at 
night. The approach of the enemy bomber was de- 
tected by the early warning system which deter- 
mined the course of the bombers. As the bomber ap- 
proached the target, it was illuminated by the inter- 
section of the beams from three searchlights (manual 
or radar controlled) which tracked it. When the ap- 
proaching bomber was detected, fighter aircraft were 
alerted and directed to the searchlight area, which 
was then patrolled at an altitude above that of the 
attacking bomber. The fighter attacked the bomber 
from above and on the tail as soon as an interception 
by three of the searchlights had been made. The 
fighter had the advantage of essentially daylight 
visibility; the bomber gunner, on the other hand, had 
impaired visibility as a result of the glare of the 
searchlight beams. The use of this technique per- 
mitted the operation of ordinary day fighters at 
night. 

Such tactics were only adequate for dealing with 
small raids under conditions of good visibility. Fur- 
thermore, concentrations of searchlights were only 
practicable near important targets, so that this de- 
fensive operation could take place only after the 
bombers had reached their objective. The need for 
radar controlled and equipped night fighters to inter- 
cept the enemy before reaching his target was recog- 
nized at an early stage of World War II, and consid- 
erable effort was expended on the development of 
such equipment. 

16.1.2 Radar Techniques 

The advent and extensive use of ground controlled 
interception [GCI] and the development of airborne 
radar systems for the detection of other aircraft re- 
sulted in great modification of night fighting tactics. 
The radar-equipped night fighters were alerted, di- 
rected by the GCI to an altitude above that of the 
raiding bomber, and then vectored (by the GCI) to 
the bomber. The GCI relinquished direction of the 


188 


fighter to the radar operator in the interceptor when 
the target was within the radar range of the fighter. 
The radar operator then directed the pilot to within 
a few hundred yards of the bomber, at an overtaking 
speed of 10 to 30 miles per hour, and to a position 
above or below (depending upon such factors as light 
conditions and clouds), and to one side of the bomber. 
After establishment of visual contact the pilot opened 
fire at the appropriate range and bearing. 

Long wave radar (approximately 200 me) sets 
were the earliest aircraft interception [AI] equip- 
ments used in the manner described above. These 
sets, developed by the British, gradually evolved 
into the Mark IV AI system (U. S. version: SCR- 
540). Later sets used shorter wavelengths (10 cm and 
3 cm) and higher power, and some provided for blind 
firing. The tabulation of AI radars in Chapter 15 
summarizes the characteristics of these later sets. 
Tactics were modified to meet the characteristics of 
each new type of set. 

16.2 GENERAL ANALYSIS OF AI TACTICS 

16.2.1 Introduction 

An analysis of AI tactics should have two major 
objects: first, to examine the tactical requirements 
with the object of making the most effective use of 
available equipment and second, to arrive at a suffi- 
ciently broad understanding of the general problem 
that more effective equipment can be designed. Al- 
though the number of possible tactical situations is 
very large from the theoretical point of view, experi- 
ence in World War II showed that the important 
general situations encountered were GCI controlled 
missions, free-lance search missions, and free-lance 
marauder missions against specific targets. 

GCI Controlled Missions 

With GCI techniques in use at the close of World 
War II the fighter could be directed from the ground 
into almost certain radar contact with the enemy. 
Failures to establish contact could usually be at- 
tributed to failures in the radar equipment or in com- 
munication. Furthermore, GCI could bring the 
fighter into contact on a favorable relative course, so 
that complicated maneuvers were not necessary for 


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GENERAL ANALYSIS OF AI TACTICS 


189 


getting into firing position, except in so far as these 
were necessitated by evasive action of the enemy. 

Free-Lance Search Missions 

The independent mission in which the fighter has 
no prior information concerning the possible location 
of enemy planes is another important tactical situ- 
ation. Two distinct stages are apparent — search 
and interception; each stage requires separate dis- 
cussion in an analysis of tactics. The interception 
stage in this situation differs from GCI interception 
in that a favorable relative course cannot be as- 
sumed. The most probable course of an enemy plane 
when radar contact is established is directly opposite 
from the fighter’s course, the condition least favor- 
able for interception. 

Free-Lance Marauding Missions 

In a free-lance mission against a known point of 
enemy air concentration such as an enemy airfield, 
search is relatively simple. The interception condi- 
tions, however, vary considerably with the exact 
situation, but in general a favorable relative course 
cannot be counted on. These night missions against 
enemy airfields were dangerous because of the neces- 
sity for maneuvering at low altitude and the accuracy 
of enemy ground fire. Many such missions were, how- 
ever, successful in World War II. 

These three tactical situations will be discussed in 
terms of two probability functions : the radar contact 
probability, which defines the probability of estab- 
lishing radar contact with an enemy; and the inter- 
ception probability, which determines the probabil- 
ity that once radar contact is established, the fighter 
can be brought into firing position. The probability 
of successful firing will not be discussed. 

Since in GCI controlled missions the fighter can be 
directed into almost certain radar contact with an 
enemy, the radar contact probability is essentially 
unity; consequently, it is not discussed further. 

16.2.2 Radar Contact Probability 

In studying radar contact probability on a free- 
lance search mission against an unknown distribution 
of enemy targets, certain simplifying assumptions 
can be made without seriously limiting the applica- 
bility of the results. It will be assumed, first of all, 
that the maximum range of the radar is large com- 
pared to the altitude range in which targets are ex- 
pected. This means that the coverage can be consid- 
ered to be cylindrical, and the problem is essentially 


reduced to two dimensions. Secondly, it will be as- 
sumed that the targets are moving at random, and 
that the average density of targets is uniform in the 
region considered. The fighter will be assumed to be 
moving with constant speed in a fixed direction. 

The analysis in the present section is largely based 
on that given by Dr. H. M. James 4 in a report dis- 
cussing the contact probability for a bomber flying 
through a uniformly patrolled region. The results 
derived in this report apply equally to the discussion 
of a fighter flying through a random distribution of 
bombers. 

A number of important results established in this 
report are summarized below with all derivations 
omitted. 

1. Most contacts will be made in the forward 
hemisphere. If u is the speed of the fighter and v the 
speed of the enemy aircraft, the per cent of contacts 
made in the forward hemisphere is given as a function 
of v/u in Table 1. It is assumed that the fighter’s 
radar has a 360-degree coverage. 


Table 1 . Per cent of forward contacts as a function 
of speed ratio (Assuming 360-degree coverage). 


V 

Per cent forward 

u 

contacts 

0.8 

92.8 

0.9 

91.0 

1.0 

89.2 

1.1 

87.2 

1.2 

84.8 

This means that little is 

gained for AI work by 

scanning the rear hemisphere, except to provide 
some tail-warning against enemy fighters. 

2. Aircraft coming into 

radar contact are most 

likely to have a relative course angle of 180 degrees 
with respect to the fighter. The distribution of these 
encounters for 30-degree intervals in the case of an 
AI set viewing only the forward hemisphere is given 

in Table 2. 


Table 2. Distribution of encounters according to 

relative course. 



Fraction of encounters 

Relative course angle 

having angle in 

(degrees) 

this range 
(per cent) 

0-30 

30-60 

2.2 

7.8 

60-90 

14.4 

90-120 

20.8 

120-150 

26.0 

150-180 

28.8 


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190 


AIRCRAFT INTERCEPTION TACTICS 


Thus 24.4 per cent will be heading away from the 
fighter and 75.6 per cent toward the fighter. These 
results are important in determining subsequent 
tactics. The independent interceptor must expect 
that most contacts will be made at large course an- 
gles, which presents a difficult problem because of 
the difficulty of turning into a homing course without 
losing the enemy. This problem will be further dis- 
cussed in Section 16.2.3. 

3. When an enemy plane is detected at extreme 
range, its most probable course is directly toward the 
observer. The nearer the bearing is to straight ahead, 
the greater is this probability. Thus, if any action is 
to be taken while an enemy is still at extreme range, 
before his course is determined, the choice of tactics 
should be made on the assumption that the enemy is 
proceeding directly toward the observer. 

Radar contact probability for a free-lance maraud- 
ing mission against a known point of enemy air con- 
centration is in general higher than for free-lance 
search missions. For, in marauder missions, the radar 
contact probability is dependent largely upon the 
ability to navigate to a fixed target where the density 
of enemy aircraft is expected to be higher. 


16.2.3 Probability of Interception 

Interception probability after radar contact is 
established is discussed here for a fixed-gun fighter 
only, since practically all interceptor aircraft used in 
World War II were of this type. The standard tactical 
procedure for this type of fighter was to approach the 
enemy from behind on an approximately parallel 
course. Interceptions are more difficult if the enemy 
is initially on an opposite course from the fighter 
when radar contact is established (as is most prob- 
able in the random search case, Section 16.2.2) than 
if accurate GCI has brought the fighter into a paral- 
lel course behind the enemy. 

James 2 has given a detailed theoretical treatment 
of the dependence of interception probabilities on the 
relevant factors involved. He makes the following 
assumptions: (1) the two aircraft are at the same 
altitude (thus reducing the problem to two dimen- 
sions) ; (2) the fighter pilot keeps his plane directed 
at the target whenever possible (homing tactics); 
(3) maximum rate of turn is used to bring the target 
dead ahead; (4) the approach is made at constant 
airspeed; (5) loss is considered to occur if the target 
gets out of the range of vision of the fighter. Some of 


the more important results of this treatment are in- 
cluded in the following discussion. Derivation of the 
results is not within the scope of this book. 

Effect of Radar Range 

For accurate GCI work the radar range need only 
be sufficiently large to ensure contact while under 
ground control. This means that the radar range 
must be greater than the GCI error radius. Greater 
range of the fighter radar will, of course, decrease the 
time which GCI must spend on a given fighter, and 
hence increase the traffic-handling ability of a given 
ground control station. This may be extremely im- 
portant in the event of large raids. 

For independent interceptions, the probability of 
successful interception increases with the range of 
the radar. Furthermore, the rate of increase of inter- 
ception probability with range is also an increasing 
function of range. The increase with range is most 
rapid when the two planes are initially approaching 
each other. 

The greater the angle of radar vision, the more 
valuable will be an increase in range. On the other 
hand, if the available rate of turn is increased, less 
stress need be laid on range. 

Speed Advantage 

Too great a speed advantage, especially in the late 
stages of pursuit, leads to a decrease in interception 
probability. As the target is approached, speed ad- 
vantage should be reduced to at most 20 per cent or 
preferably 10 per cent. 

As the radar range is decreased, there is increased 
danger of losing the target if the speed advantage is 
too great. 

Available Rate of Turn 

For GCI interception, the rate of turn is princi- 
pally important in those phases of the pursuit when 
the target is taking evasive action. 

For independent interceptions, where the initial 
relative course angle can be expected to be large, the 
probability of a successful interception increases 
rapidly with the maximum rate of turn of the fighter. 
In night fighting there is a tendency for pilots to re- 
strict themselves to relatively low turning rates; this 
should be avoided so far as possible. However, the 
advantage of increased rate of turn falls off rapidly 
as radar range is increased. 


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SPECIAL PROBLEMS IN AI TACTICS 


191 


Angle of Radar Vision 

For GCI interceptions a large angle of vision is 
valuable both in ensuring radar contact, and in fol- 
lowing evasive action. 

Wide-angle vision is even more important in the 
case of independent interceptions where unfavorable 
initial courses can be expected. Increase in angle be- 
yond 180 degrees is probably not worth the added 
weight required, but increase from 120 to 180 degrees 
greatly improves the chance of interception. 

Persistence of Pursuit After Target Disap- 
pearance 

The target sometimes will go out of the range of 
vision (in angle) of the pursuer at some stage of the 
pursuit. This situation is most likely when a small 
rate of turn is available. In this case it is very im- 
portant that the pursuer not give up, but continue in 
his circular course (at maximum rate of turn). Such 
persistence may add as much as 30 per cent to the 
probability of interception. 

The greater the available radar range, the more 
important is this persistence. The greater the avail- 
able rate of turn, the smaller will be the increase in 
target range during the time that the target is out of 
sight. 

16.3 SPECIAL PROBLEMS IN AI 
TACTICS 

In Section 16.2 AI tactics were discussed from a 
general point of view. In this section we will discuss 
the effect of certain special factors on tactics. 

16.3.1 Firing Methods 

Once a fighter is on the tail of an enemy aircraft 
either visual firing or blind firing (if the radar is 
equipped with this facility) may be used. Blind firing 
has several advantages over visual firing: (1) the 
range information is much better than any visual 
estimate, especially when made at night; (2) radar 
firing can begin at greater ranges, thus reducing the 
danger of detection and defensive fire by the bomber; 
(3) the total time of approach is decreased; (4) the 
angle about the tail of the enemy aircraft, in which 
night fighters are expected at any stage during their 
approach, is increased ; (5) the turning rates required 
to keep the night fighter’s sights on the bomber are 
decreased. Visual firing, on the other hand, has the 
big advantage of better target identification, but this 


factor is much less vital if adequate radar identifi- 
cation equipment is available. 

AI radars, such as the AN/APS-6, which have 
blind firing directed by a conical scan, restrict the 
final closing operation to a direct tail chase. The con- 
dition that the target be centered on the G scope, and 
remain centered, is that the line of the fixed guns of 
the fighter (and thus of the conical scan axis) remains 
pointed at the target. Enemy evasive action is very 
likely to be successful against this type of installation. 

Night fighters equipped with turrets such as origi- 
nally planned for the P-61 would, of course, allow 
much more flexible tactics in the closing stages of 
pursuit than were possible with fixed gun fighters. 
Automatic following, such as provided by the AN/- 
APG-1 system, allows easier following of evasive 
action than is the case with nontracking radar sets. 

16.3.2 Nature of Target 

Tactics should be planned with all possible knowl- 
edge of the enemy target in mind. The most impor- 
tant factors to be considered are: the coverage and 
range of defensive radars (such as tail- warning) ; the 
nature and extent of enemy defensive fire power; the 
speed of the enemy planes; and the probable tactics 
which the enemy will use in avoiding pursuit. Prior 
knowledge of any of these factors will affect both 
GCI and AI techniques. 

The tactics employed in combating missiles, of the 
guided or unguided type, depends upon the type of 
missile. Missiles such as the German V-l buzz-bomb 
are easily visible at night, so that radar-equipped 
night fighters are not required. The tactic adopted in 
this case consisted of cruising at an altitude several 
thousand feet higher than that at which the V-l’s 
were flying; when a buzz-bomb was detected, the 
fighter dived toward the missile, thereby obtaining a 
speed advantage. 

16.3.3 Evasive Action 

The evasive action frequently employed by bomb- 
ers is to change altitude and course (by 5 or 10 de- 
grees) at intervals of 1 to 2 minutes. Thus, the weav- 
ing character of the target course may cause diffi- 
culty for a night fighter in the final stages of ap- 
proach. An evasive maneuver very difficult for the 
night fighter to follow is a sudden sharp turn through 
360 degrees by the bomber to throw the night fighter 
off the bomber’s tail. This maneuver is especially 


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192 


AIRCRAFT INTERCEPTION TACTICS 



difficult to follow if the fighter is close to the bomber 
at the time the bomber initiates this evasive action. 
Another maneuver which is very effective is to peel 
off in a steep dive to left or right. In general, how- 
ever, large angle turns will not be encountered by the 
defending fighter, because a bomber employing such 
tactics would require extra fuel at the expense of 
bomb load. 

16.3.4 Effect of Ground or Sea 

Clutter and Countermeasures 

The quarry can be lost during the approach be- 
cause of disappearance in the altitude signal, dis- 
appearance in the ground or sea clutter, or the effects 
of window or chaff. A discussion of the altitude sig- 
nal and sea and ground clutter is given in Sec- 
tion 15.1.6. References 9 and 10 contain a detailed 
discussion of sea return and altitude effects in the 
AN/APS-6 system, with particular reference as to 
how the tactics might be affected for this type of 
system. 

Window or chaff is sometimes used to protect at- 
tacking bombers against defending interceptors. The 
object of using window is to present the equivalent of 
a large number of reflecting dipoles so as to effec- 
tively jam the GCI and AI radars protecting the 
territory under attack. Window is dropped from 
several of the attacking aircraft in an attempt to con- 
fuse the GCI and AI radars. Narrow beam and short 
pulse durations in the radar systems are the most 


effective means of penetrating this interference, al- 
though it is possible, in principle at least, to drop 
sufficient window to cause loss of the target alto- 
gether. 

16.4 THE RANGE CLOCK 

In any interception the night fighter must get onto 
the course of the bomber and convert the approach 
to a tail chase in the most efficient manner possible. 
James 6 has devised a range clock which enables a 
night fighter to get on the course of the target with a 
minimum number of radar observations. The use of 
the range clock reduces the approach to a target to 
the following steps: (1) turning until the target is 
dead ahead, (2) flying a straight course at a standard 
airspeed, and (3) executing a single turn at a standard 
rate through an angle determined in advance. 

A range clock, in its simplest form (Figure 1), con- 
sists of a moving hand, a marker having a fixed po- 
sition during each run, and two scales on the dial 
face. The outer scale is a range scale calibrated in 
miles; the inner scale is calibrated in degrees and in- 
dicates the point at which the turn is to start. For 
application to large angle interceptions two addi- 
tional markers are added to the range clock, produc- 
ing the so-called standard range clock (Figure 2). 
These two additional markers allow the radar oper- 
ator to determine whether the interception is nearly 
a head-on interception, and provide a method of 
hand lingsuch interceptions. 


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THE RANGE CLOCK 


193 


The operation of the simple range clock is begun 
with both hand and marker in the zero position. 
When the starting knob (upper left corner) is pulled 
out the hand can be set to any range indicated on the 
outer scale. The marker is automatically set at one- 
half of this range. 

Referring to Figure 3 we will assume that the fighter is at P 
after having turned toward the target which is now dead 
ahead at range PT. The clock is set at range PT and started; 
the pilot flies a straight course (P to T ). As described in 



Figure 3. Approach of fighter to target using range 
clock procedure. 

James’ report, “the hand of the range clock will move toward 
smaller ranges at a rate equal to the standard airspeed agreed 
on in advance for use in these approaches; thus it will indicate 
at any instant the distance still to be traversed before the 
point T is reached. 

“When the clock is started the marker remains fixed. When 
the hand has reached the marker the pursuing plane will be at 
point 0 (Figure 3), halfway to point T , while the target will be 


at O', nearly an equal distance from T. At this moment the 
radar operator will observe the bearing angle 0 to the target. 

“By doubling the angle /3 the radar operator can determine 
the angle between the courses of the two planes, which is the 
angle through which the pursuing ship must turn. He should 
then at once pass this information to the pilot.” 

Doubling the bearing angle to obtain the angle be- 
tween the courses is exact if the speeds of the target 
and pursuer are equal ; if not, correction may have to 
be made for greater precision. 

To determine the time at which this turn should be 
begun, the radar operator will now refer to the inner 
scale. This is calibrated in terms of the angle between 
the courses of the planes; the calibration depends on 
the rate of turn to be used. When the moving hand 
reaches the calibration corresponding to the angle 2/3 
already determined, the turn should be started as 
soon as possible. In practice there will be a lag in the 
actual starting of the turn; for best results the radar 
operator should call for the turn somewhat before the 
hand actually reaches the calibration. The correct 
amount can be learned by experience. 

The simple form of range clock procedure described 
above is effective, except when there is a very large 
angle (greater than 130 degrees) between the courses 
of the planes, and an appreciable speed difference. 
In order to deal adequately with all interceptions at 
very large angles, two more markers must be added 
to the range clock, thus producing the standard range 
clock (Figure 2). Use of this standard clock is fully 
described in James’ report. 6 


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PART IV 


AIRBORNE FIRE-CONTROL RADAR 


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X V 








* 
















Chapter 17 

GENERAL CONSIDERATIONS ON AIRBORNE FIRE CONTROL 


17.1 INTRODUCTION 

This part, consisting of the present chapter and the 
following five chapters, contains a discussion of vari- 
ous fire-control radar systems developed during 
World War II, with some suggestions for future de- 
velopments. The treatment is not complete for all 
systems, and some have been omitted altogether. 
Certain systems chosen for their representative na- 
ture have been discussed in considerably more detail 
than others. Although the emphasis is on systems 
developed at the Radiation Laboratory, others are 
included to give balance to the picture, especially 
where the first system of a given type was developed 
at the Radiation Laboratory, while the responsibility 
for modifications and refinements (sometimes quite 
extensive in nature) was turned over to manufac- 
turers working under Service contracts. 

17.2 OTHER NDRC FIRE-CONTROL 

AGENCIES 

The Applied Mathematics Panel, under the di- 
rection of Dr. Warren Weaver, and Division 7 of 
NDRC, headed by Professor H. L. Hazen, were con- 
cerned with general fire-control developments during 
World War II. Reference should be made to Vol. 2, 
Part A of the Summary Technical Report of the 
Applied Mathematics Panel, written by Dr. E. W. 
Paxson, 88 and to similar reports for NDRC Sec- 
tion 7.2 (Airborne Fire Control) by Drs. J. B. Russell 
and G. A. Philbrick. 12 ’ 13 Section 7.2 further allowed 
its contractors to submit final reports consisting of 
their interim reports bound together with suitable 
introductions. One of the Applied Mathematics 
Panel agencies most active in the field of airborne 
fire control was the Applied Mathematics Group at 
Columbia University under the direction of Pro- 
fessor S. MacLane. A survey of this work has been 
published, 90 as well as a bibliography with abstracts 
of some of the papers. 117 

17.3 CLASSES OF RADAR SYSTEMS 

Fire-control radar systems fall into three general 
categories. These are discussed in Chapters 18, 19, 
and 20. Chapter 18 deals with automatic following 
equipment, that is, radar systems which first locate 
and then automatically track and range on a target 


aircraft. Sets of this kind are the most complicated 
fire-control radar systems; they are so complicated 
that none were in service by the end of World War II, 
except for experimental installations in a squadron 
of night fighters which reached the theater too late to 
see combat. (The radar sets themselves were coming 
off the production line in the spring of 1944, but in- 
stallation was held up pending decisions on related 
computers and tactical needs.) 

Chapter 19 discusses an intermediate kind of 
equipment which provides for automatic ranging and 
gives the gunner a scope indication of target position 
which he can use for tracking. This equipment was 
simpler than the automatic following equipment and 
problems of development and installation (including 
computer tie-in) were solved in time for its use in 
considerable quantities during World War II. 

Chapter 20 treats fire-control radar systems which 
supply only range-to-target information. Some sys- 
tems are designed for air-to-air ranging, some for 
air-to-ground ranging, and others for both. Some of 
these systems are automatic in operation, others re- 
quire a radar operator, and all require optical aiming. 
The historical aspects of airborne range-only radar 
are discussed in a memorandum 53 by A. F. Sise, 
chief of the airborne fire-control section of the Radi- 
ation Laboratory. 


17.4 PROBLEMS IN THE DEVELOP- 
MENT OF AIRBORNE FIRE- 
CONTROL RADAR 

The role of radar is to supply information about 
the target. To be of any use this information must 
somehow contribute to gun pointing. During aerial 
combat a gunner does not have time to read the 
radar information from dials and scopes, consult fir- 
ing tables to determine correct aiming point, then 
point his guns and finally fire. The total duration of 
a nose attack against a fast bomber is frequently less 
than two seconds and even a tail attack is unlikely 
to last more than 20 seconds. Even on the slower 
attack the correct aiming point changes rapidly with 
time, so use of tables during flight is impractical. 
Thus, there must be some kind of computer to trans- 
form the radar-supplied information about the target 
into gun pointing. Some of the problems encountered 

197 



GENERAL CONSIDERATIONS ON AIRBORNE FIRE CONTROL 


in integrating radar into a fire-control system are 
discussed in Chapter 21. 

A difficult phase of the development of airborne 
equipment is assessment. This is important both in 
the engineering stages and later in Service acceptance 
tests. A considerable amount of effort went into de- 
veloping suitable techniques. Assessment problems 
are discussed in Chapter 22. 

After a radar set has been designed and accepted 
by the Services its proper functioning and use must 
be assured. Training problems both for maintenance 
and operation are of primary importance. See Chap- 
ter 13 for a discussion of this topic. 

17.5 REASONS FOR SLOW PROGRESS 
OF AIRBORNE FIRE-CONTROL 
RADAR SYSTEMS 

In spite of the very considerable amount of work 
done in developing airborne fire-control radar sys- 
tems, very few of them saw combat use. A discussion 
of reasons for this may be useful in planning future 
developments, and in preventing a repetition of the 
mistakes of World War II. Although the points listed 
below are not all independent of each other, it is the 
authors’ belief that they are sufficiently different to 
warrant separate listing. 

17.5.1 Lack of Overall System 

Responsibility 

This was in the authors’ opinion the main cause of 
delay. Coordination between sight designers and 
radar designers left much to be desired. This lack of 
coordination was not helped by the NDRC assign- 
ment of sight development to one Division and radar 
development to another Division. A typical attitude 
of a sight designer was that his sight would be ready 
long before any corresponding radar, so he was not 
greatly concerned with making provision for possible 
radar inputs. On the other hand the radar systems 
were frequently designed without much thought 
about related sights. This point is treated in more 
detail in Chapter 21, in particular in Sections 21.3.3 
and 21.3.4. 

17.5.2 Delays Caused by Accept- 

ance Tests 

So many new gadgets were thrust upon the testing 
authorities of the Army and the Navy that any par- 
ticular item was subject to numerous delays, unless 


it had some special push from higher up. Many radar 
sets by-passed the test agencies, but this was not true 
in the airborne fire-control field. Further discussion 
of this point is given in Chapter 22. 

17.5.3 Delays Due to Training 

Problems 

Complicated equipment such as a radar system re- 
quires expert maintenance and a well-trained oper- 
ator. This is true even at the test level. But here a 
vicious circle was encountered. Under Army policy a 
training device for a piece of equipment could not 
have high enough priority to permit its manufacture 
until the latter had been approved by the test agency. 
The situation for maintenance training was similar. 
Thus, after test approval was given, the system still 
had to wait for the training of maintenance men and 
operators. This point is discussed further in Chap- 
ter 22 with respect to its influence on the test pro- 
gram. 

17.5.4 Delays Due to Slowness of 
General Airborne Fire-Control Program 

This is related to the first reason listed above. A 
widespread attitude in the Services was that the aim- 
ing of guns was one place in which civilian scientific 
help was not needed. Although the staff of the Secre- 
tary of War included several “expert consultants” on 
radar and other recognized technical developments, 
it was not until the late spring of 1945 that an expert 
consultant in the airborne fire-control field was 
added. Further discussion of this point will be found 
in various parts of Chapter 21. 

17.5.5 Delays Due to Overam- 

bitious Programs 

The first development of radar for airborne fire 
control was in automatic following equipment. When 
it became evident how much time completing such a 
program would take, the more ambitious projects 
were tabled and work was begun on simple light- 
weight systems. This trend is illustrated by the early 
emphasis on system AN/APG-1, an automatic gun- 
laying radar set weighing about 700 lb and the much 
later initiation and development of AN/APG-13, a 
set weighing less than 100 lb, designed to provide 
range information for isolated water targets, and 
which incidentally was one of the few airborne fire- 
control radar sets successfully used in combat. This 



PROGRESS OF AIRBORNE FIRE-CONTROL RADAR SYSTEMS 


199 


point was important as a reason for delay but is prob- 
ably not of great importance for future develop- 
ments, especially during peacetime conditions. 

17.5.6 Delays Due to Changing 
Tactical Conditions 

By the time lightweight practical radar sets were 
available for airborne fire control the tactical situ- 
ation had so changed that they were no longer so 
urgently needed as earlier in the war. In the last few 
months of the war losses from enemy fighter action 
dropped to such an extent that combat commands 
were no longer pleading for defensive equipment. 
This resulted in lack of pressure throughout the 
whole testing and training programs. If combat losses 
had remained high there would undoubtedly have 
been considerable use of some of the airborne fire- 
control radar systems. Even in the case of Falcon 
(AN/APG-13), which did see successful combat use 
against river boats in China, further advances of the 
Japanese almost neutralized the equipment by push- 
ing AAF bases out of practical combat range from 
suitable targets. The later more complicated sys- 
tems AN/APG-13B and AN/APG-21 would have 
been useful here but were not developed soon enough. 


17.5.7 Delays Due to Limited Uses 
of Airborne Fire-Control 
Radar Systems 

Airborne fire-control radar systems do not have the 
variety of uses encountered in many other airborne 
systems; consequently the fire-control program did 
not have as strong support as other airborne radar 
programs. If the fire-control systems had weighed 
only a few pounds the lack of other uses would not 
have been so important, but it was quite understand- 
able that the Services hesitated to install an un- 
proved system which weighed as much as a man or 
as a small bomb, even though the new equipment, if 
successful, might save several times its own weight 
by reducing the amount of ammunition needed. Al- 
though there may be a few peacetime applications 
for fire-control radar sets (such as anticollision or 
point-to-point communication), it does not seem 
that these will be sufficient to stimulate commercial 
developments. This places a strong obligation on the 
Services to push radar developments in the airborne 
fire-control field by their contractors and by their 
research laboratories. 


RI 


ESTRIGTED 


Chapter 18 

AUTOMATIC FOLLOWING EQUIPMENT 


18.1 OPERATION 

18.1.1 Function of Equipment 

In an airborne gunlaying [AGL] radar system, the 
scanner or antenna tracks the target airplane contin- 
uously and automatically in azimuth, elevation, and 
range. It searches for a target through a fairly large 
volume in space, gives the operator angular position 
and range information on all targets in that volume, 
and operates with special equipment to identify those 
targets as friend or foe without optical contact. It can 
be quickly and simply locked on the desired target in 
angle and in range, and will thereafter automatically 
follow the target in those coordinates. The necessary 
fire-control data are fed from the radar to a comput- 
ing mechanism which may be either a lead-computing 
sight or a central-station type of computer. The com- 
puting mechanism controls the aiming of turret guns. 

In addition to the functions mentioned above, a 
specific navigational aid, beacon interrogation, is in- 
corporated in night-fighter equipment. AGL can be 
used offensively in night fighters against attacking 
bombers or for intruder work. It is used defensively 
in bombers for tail protection. 

In the present chapter, only those systems designed 
in the United States are discussed. There are some 


important British developments as well, of which 
some features are mentioned. 

There are at present two fundamental design types 
of systems, illustrated in Figures 1A and IB. In one, 
the antenna is mounted directly on the turret (e.g., 
AN/APG-16, AN/APG-19); in the other it is sep- 
arately mounted (e.g., AN/APG-1, AN/APG-2, 
AN/APG-3). Both must fulfill similar search require- 
ments, but the former need only track through an 
angle equal to the maximum lead angle whereas the 
latter must be capable of tracking through a solid an- 
gle even greater than that needed for search alone. 

18.1.2 Radar Boresight 

A conical-scanning antenna, used by all present 
microwave gunlaying systems, or its equivalent, a 
lobe-switching antenna, is the heart of the system 
around which all other components are designed. The 
antenna has a so-called radar boresight line or radar 
boresight direction which is defined almost as closely 
as an optical sight line. This radar boresight direction 
is obtained by rotating the main lobe of radiated 
power about an axis which is offset several degrees 
from the axis of symmetry of the lobe. 

A target lying on the radar boresight direction will re- 
turn the same power from each successive pulse except 


MODULATOR 

PULSE 

R-F HEAD 

R F 

T-R SWITCH 

R F 

ANTENNA 















SYNCHRONIZING 

PULSE 


SYNCHRONIZER 


SYNCHRONIZING PULSE 


AUTOMATIC 

RANGE 

TRACK 




ANTENNA SERVO 

AMPLIFIER AND 

MOTOR DRIVES 

POSITION 



RECEIVER 


DRIVE 












LEAD ANGLE 
SIGNAL 


COMPUTING 

SIGHT 


ERROR 

SIGNAL 


TURRET 

POSITION 

DRIVE 


TURRET 

POSITION 

SIGNAL 


TURRET SERVO 
AMPLIFIER AND 
MOTOR DRIVES 


Figure 1A. Block diagram of an AGL radar system with the antenna mounted on a turret and operating with a com- 
puting sight in the turret. 


200 


RESTRICTED, J 







OPERATION 


201 



Figure IB. Block diagram of an AGL radar system operating with a director type of computer not located in the 
turret. (There should be a line from the automatic range track box to the central station computer, indicating range 
input.) 


for fluctuations caused by fading and propeller modu- 
lation, and by target polarization effects if the spin- 
ner includes a rotating dipole. A target lying off the 
boresight axis, as illustrated in Figure 2B, will return 
power modulated at the conical-scan frequency and 
its harmonics, the latter in amounts depending upon 
antenna design. 22 The phase and amplitude of the 
fundamental frequency are determined by the angu- 
lar position of the target in relation to the radar bore- 
sight line of the antenna. 

18.1.3 Automatic Following 

When the system is on track (follow), the signal 
from the desired target is automatically tracked in 
range. The output of the tracking gates, which con- 
tain video information relative only to the desired 
target, is fed into a third detector. The output of the 
third detector is the error signal, that is, the audio 
envelope of the power received from the desired tar- 
get. This error signal, when amplified, is fed into a 
commutator circuit wherein it is compared with con- 


ical-scan reference voltages obtained from a gener- 
ator attached to the antenna. The commutator cir- 
cuit provides d-c voltages, the polarity and magni- 
tude of which depend upon the direction and amount, 
respectively, of the error in the azimuth and elevation 
channels. These d-c voltages are then applied to am- 
plidynes or similar devices which drive the scanner or 
turret in such a manner as to reduce the error signal 
to zero. 

18.1.4 Computation 

Selsyns or potentiometers transmit azimuth and 
elevation information to the computer or to the com- 
puting sight as the case may be, and rate gyros on the 
scanner, on the turret, or in the sight transmit angu- 
lar rates. The range unit delivers a d-c voltage to the 
computer. These data plus ballistic input data de- 
termine the lead angle. When the scanner is mounted 
separately from the turret, selsyn linkage between 
turret and computer provides the signal to the turret 
servo-amplifier which causes the turret guns to track 




ESTRICTED 




202 


AUTOMATIC FOLLOWING EQUIPMENT 


BEAM UP 



ERROR 



B C 


ECHOES WHEN TARGET IS OFF ECHOES WHEN TARGET IS ON 

THE RADAR BORESIGHT AXIS THE RADAR BORESIGHT AXIS 

Figure 2. Conical scanning with rotating reflector. 

with the necessary lead angle. When the scanner is 
mounted on the turret, the computing sight delivers 
a signal to a servo-amplifier which deflects the scan- 
ner from the gun boresight line by the amount of the 
computed lead angle. 

The radar scanner must be harmonized with the 
guns so that the radar boresight line and the gun 
boresight line are parallel when no lead is given by 
the computer. 

18.1.5 Search 

There are several methods of performing the search 
function. The simplest and most common is a type of 
Palmer scan illustrated in Figure 3 A. An illustration 
of the search coverage of a type of system used in 
offense is given in Figure 3B. The antenna persists in 
a conical scan, but in addition the antenna mount is 
rotated alternately in azimuth and in elevation, per- 
forming a closed loop. The motion may include one or 
more steps in elevation per cycle. This motion is gen- 
erally accomplished by means of cams and micro- 
switches which direct power to the azimuth and ele- 
vation drives in the proper sequence and duration. 

18.1.6 Track 

When switching from search to track, the operator 
must close an action switch which gives him control 
of the scanner position. Then, while positioning the 
scanner for maximum return from the target (search- 
lighting) as indicated on his scope, the operator 
throws a search-track switch to track, and either puts 


a range-tracking gate manually on the target echo 
(as in AN/APG-1, AN/APG-2) or the system auto- 
matically locks on the target (as in AN/APG-3, 
AN/APG-16, AN/APG-19). With the latter the op- 
erator can press a range-gate in-out switch, until the 
gate locks on the desired echo, if there is more than 
one. Following release of the action switch, the scan- 
ner automatically follows the target. When the action 
switch is closed, the operator has complete control of 
scanner pointing regardless of the position of the 
search-follow switch. 

18.1.7 Identification 

Automatic following equipment is designed to op- 
erate with equipment to identify a target as friend or 
foe (IFF equipment). 57a In one type of IFF system 
the radar supplies a trigger to the IFF system and the 
latter transmits an r-f pulse at the same time that the 
radar r-f pulse is transmitted. If the target is friendly, 
it has equipment which will send back a coded reply 
which is received and converted into video by the 
IFF system. The radar operator wishing to identify 
the radar echo merely searchlights the scanner on the 
target and switches from the video presentation of 
his own set to that fed it by the IFF system. 

18.2 REQUIREMENTS 

The requirements placed upon the radar depend in 
some measure upon the tactical use to which the 
radar is to be put. These uses may be offensive as in 
a night-fighter radar, or defensive as in a bomber 
defense radar. 

18.2.1 Requirements Common to 
Defensive and Offensive Radars 

1. The set must be light and compact. 

2. It must meet certain specifications on altitude, 
temperature, and radio noise. 

3. The set must directly (or indirectly through 
associated equipment controlled by it) provide the 
computing device with target azimuth and elevation, 
azimuth and elevation rates, and range. 

4. The set must be able to track the target air- 
plane in azimuth, elevation, and range with a high 
degree of accuracy under conditions of maneuvering 
and of low-level attack, through a large volume in 
space. The accuracies required depend upon com- 
puter requirements, target characteristics, and the 
characteristics of the firing device. The specifications 


RESTRICTED f 


LEFT LIMIT 


REQUIREMENTS 


203 


APPROXIMATE BOUNDARY OF AREA SEARCHED 



APPROXIMATE BOUNDARY OF AREA SEARCHED 

Figure 3A. Palmer-scan method of searching represented on a flattened surface. 








204 


AUTOMATIC FOLLOWING EQUIPMENT 


stipulated by the Services for sets AN/APG-3 and 
AN/APG-16 called for probable errors of not more 
than + 34 degree in azimuth and in elevation, and 
not more than + 25 yd in range, when the tracking 
rate is less than 22.5 degrees per second, and the ac- 
celeration is less than 20 degrees per second per 
second. 

5. The desirable minimum tracking range is 75 to 
100 yd. 

6. Search periods must be short because of the in- 
creased flexibility of attack approaches. If a single 
scanner performs both search and track functions, 
the switchover must be rapid. 

7. The set must provide means of positive identi- 
fication of the target plane as friend or foe so that 
optical contact with the target is not necessary. 

8. The Services have stipulated that in daytime 
operations where tracking is done optically, the 
equipment should supply only range. 

18.2.2 Requirements Pertinent 
Only to Offensive Radar 

Night fighters must be able to contact enemy in- 
truder planes with or without the aid of ground con- 
trolled interception [GCI]. They should further be 
able to detect enemy bombers out to 20,000 yd or 
better. The system must have large search and track 
coverages. (Coverage of AN/APG-1 and AN/APG-2: 
search +82.5 degrees in azimuth, and from — 12.5 to 
+ 17.5 degrees in elevation; track +82.5 degrees in 
azimuth, and from — 12.5 to +80 degrees in eleva- 
tion.) A large tracking coverage is desirable to permit 
flexibility in approach and efficient utilization of fire 
coverage. The desirable search period is a debatable 
point depending largely upon the angular limits of 
the volume covered. A period of 2 or 3 sec should be 
short enough to satisfy all present tactical require- 
ments. 

An indication of target position, range, and closing 
speed must be given the pilot to facilitate contact 
and to permit him to home on the target and to fire 
his fixed guns as well as the turret guns, if he so de- 
sires. It also enables him to follow a maneuvering or 
“jinking” target. 

18.2.3 Requirements Pertinent 
Only to Defensive Radar 

An ideally protected bomber should have fore and 
aft gunlaying radars which give spherical coverage in 
search. Track coverage should be somewhat better 


than the turret coverage to permit full utilization of 
the turret fire power. Nose coverage range should be 
greater than tail coverage range. With a closing rate 
of 800 mph, a detection range of 20,000 yd would be 
needed in order to give 50 sec warning. At the end 
of World War II, 5,000 yd was considered adequate 
for tail protection. The use of guided missiles, faster 
fighters, rockets for air-to-air combat equipped with 
proximity fuses, or new tactics may necessitate 
greater coverage. 

When search ranges become greater than the alti- 
tude at which the aircraft is flying, it will be neces- 
sary to provide adjustable limits on the automatic 
range-search gate or the equivalent to prevent it 
from locking on the altitude signal; some special 
means of preventing ground echoes from obscuring 
air targets should also be provided (see “Indication,” 
Section 18.6.1). In doing this, it is necessary to re- 
member that the operator is primarily a gunner, and 
consequently the number of controls should be kept 
as small as possible. 

It is highly desirable to be able to search and track 
at the same time. This requirement may necessitate 
separate systems for search and for track. A compro- 
mise such as a system which could be momentarily 
switched from track to search and back again with- 
out interrupting the continuity of computing data 
might suffice. 19 

18.3 DESIGN CONSIDERATIONS 

18.3.1 Wavelength 

Choice of wavelength is dictated primarily by con- 
siderations of power requirements, absorption, sus- 
ceptibility to jamming, and antenna requirements. 
Other factors, as yet not completely evaluated, also 
may affect the choice of wavelength. These include 
the influence of refraction by jet gases, and the effect 
of the reduction of the cross-sectional area of the 
target, as in ultra-streamlined jet-propelled aircraft 
and guided missiles. 

Antenna Considerations 

The design of antennas for fire-control systems 
presents problems very similar to those encountered 
in the design of bombing and AI systems, which are 
discussed in Chapter 7 (especially in “Factors Affect- 
ing Radar Range,” Section 7.2.2) and in Chapter 15, 
respectively. Thus, in changing from S to X band it 
was found possible to make a considerable reduction 


^RESTRICT 



DESIGN CONSIDERATIONS 


205 


in weight and in size. AN/APG-1 and AN/APG-2, 
S band sets, weighed approximately 600 lb exclusive 
of the weight of the mounting brackets, whereas 
AN/APG-3 and AN/APG-1 6, X band sets, weighed 
approximately 225 lb installed. (These figures do not 
include the weight of the a-c power supply.) This re- 
duction resulted from increased knowledge of the art, 
better packaging, the use of smaller r-f components, 
and decreased antenna size. A further reduction, 
though not so great, would be expected at K band. 
It might well be possible to design two K band an- 
tennas, one a continuously searching antenna, the 
other a tracking antenna, perhaps with an r-f switch 
between the two, so that only one modulator and r-f 
head would be required. Together these might ap- 
proximate the size and weight of a single X band an- 
tenna, and at the same time meet the search-track 
requirements laid out in Section 18.2. Antenna de- 
velopment is discussed further in Section 18.6.1. 

Other Considerations 

Preliminary investigations indicate that the index 
of refraction of hot jet gases is very small. If, how- 
ever, an AGL antenna is located very near a jet ex- 
haust and the radar boresight axis is in the region of 
grazing incidence with the direction of flow of the jet 
gases, harmful refraction of the radar beam can take 
place, causing the antenna to be mispointed. 

Proper streamlining has been found to reduce the 
effective cross-sectional area of a target to micro- 
waves. Hence, an ultra-streamlined jet-propelled 
fighter airplane may be extremely difficult to detect 
with microwave radar. (The absence of propellers 
also has an effect.) The effect of streamlining in re- 
ducing the effective target cross-sectional area is 
greater at these very short wavelengths than at 
longer ones. 

18.3.2 Antenna 

The Scan Function 

A conical scan or its equivalent is required to de- 
termine the angular position of the target. There are 
two principal methods of obtaining this conical scan. 
A parabolic reflector may be mechanically offset from 
the feed axis by approximately half the desired beam 
offset angle and the reflector may be rotated around 
the feed axis as shown schematically in Figure 2. 
AGL systems using this type of antenna are AN/- 
APG-3, AN/APG-16, and AN/APG-19. Such an 
antenna is feasible for systems operating at X or 


K bands, but with S band the large size required for 
the reflector causes mechanical difficulties. Early de- 
velopment models at S band used a rotating feed 
which was mechanically offset from the paraboloid 
rotation axis. 

The second method, in which the feed and the axis 
of the parabolic reflector are coincident, is electro- 
mechanical. The main lobe of radiated power is offset 
from this principal axis electrically by use of a choke 
located behind the dipole. The distance between the 
choke and the disk reflector at the end of the feed is 
chosen to cause resonance at the radio frequency 
used. The phase of the energy radiated by the out- 
side of the feed between the choke and the disk re- 
flector combines with that radiated by the dipole 
after reflection by the paraboloid in such a manner as 
to cause a shift in phase of the wave front, offsetting 
the beam from the axis of the paraboloid (and feed) 
by the desired amount. 21 Mechanical rotation of the 
feed produces the conical scan. This method is well 
suited for S band and is used in the AN/APG-1 and 
AN /APG-2 systems. It is shown in Figure 4. Rotat- 



Figure 4. Electrical-offset antenna feed, for conical 
scanning (see also Figure 10). 


ing the feed presents certain problems in design not 
found in other types of conical-scan mechanisms. 
Since S band wave guide is inconveniently large, it 
is necessary to use coaxial feed, and careful attention 
must be paid to the design of the coaxial high-speed 
rotary joint. A poorly designed joint will cause the 
impedance facing the magnetron to change as the 
feed rotates, thereby causing the magnetron to 
change frequency (frequency pulling) at the conical- 
scan rate. Frequency pulling at the conical-scan rate 
introduces spurious signals at the error signal fre- 
quency and its harmonics, 47 which result in the mis- 
pointing of the antenna and in erratic tracking, un- 
less steps are taken to nullify its effect. These spuri- 
ous signals arise from the unequal action of the rela- 


206 


AUTOMATIC FOLLOWING EQUIPMENT 


tively narrow band-pass of the i-f strip upon the dif- 
ferent frequencies present. The effects of this fre- 
quency pulling can be lessened by a fast-acting auto- 
matic frequency control [AFC] — an i-f strip several 
times wider than would be required in the absence of 
frequency pulling — and by a broad-banded du- 
plexes (The duplexer is that portion of the r-f feed 
which interconnects the magnetron, output coaxial 
line, and transmit-receive [TR] box.) A properly de- 
signed AFC will reduce the frequency modulation of 
the i-f signals to a tolerable amount ; amplitude mod- 
ulation of the signal caused by changes in interme- 
diate frequency will be smaller for a broad i-f strip 
than for a narrow one; and a broad-band duplexer 
will reduce the amount of frequency pulling of the 
magnetron caused by changing impedance at the an- 
tenna. At least two of these precautions are desirable 
to provide adequate protection against the harmful 
effects of faulty operation of the high-speed coaxial 
rotary joint. 

The signal obtained with this type of antenna is 
not so good as that obtained with the rotating offset 
paraboloid. The r-f polarization rotates with the feed, 
and any degree of polarization of the target intro- 
duces into the signal undesirable components at har- 
monics of the conical-scan frequency. It also intro- 
duces variable phase shifts in the fundamental fre- 
quency (maximum approximately 30 degrees) and 
in its harmonics. These shifts can increase the time 
required for synchronization, which is the time re- 
quired for the system to lock on the target, and, after 
synchronization, can cause the antenna to spiral 
around the target. Similar phase shifts occur with 
rotating paraboloid antennas but to a considerably 
smaller degree. 22 

Most other scanning methods are untried and only 
one is discussed in this section. 

Electronic switches have been devised which 
enable the switching of power from one feed to an- 
other. 27 - 31 An antenna with four feeds (corresponding 
to beam up, right, down, left) activated by such 
switches in the proper sequence would give the equiv- 
alent of a conical scan. 

The Sperry AGL-2 system, operating at X band, 
obtained the conical scan by rotating the feed and 
reflector together. Their common axis was offset from 
the axis of rotation by a small angle. The General 
Electric pantograph scanner, operating at X band, 
utilized a mechanically offset rotating feed to obtain 
the conical scan. 

The speed of conical scan affects the operation of 


the system in several ways. There are frequency com- 
ponents in the error signal which enter from sources 
beyond the control of the designer. It is desirable, of 
course, to choose a conical-scan frequency that will 
fall outside of the bandspread of these undesirable 
but unavoidable frequencies. These frequencies are 
caused by propeller modulation, 49 which is an ampli- 
tude variation of the return echoes caused by the 
cyclic change in reflection by the target propeller 
blades. They also result from fading of the target 
signal strength. The presence of these signals at the 
conical-scan frequency and, with certain types of com- 
mutator circuit, at its harmonics will cause improper 
tracking. Since high conical-scan rates are desirable 
for the sake of servo stability and speed of response, 
it would be best to choose a conical-scan rate above 
the bandspread of propeller modulation and fading 
frequencies. However, this high rate has not been 
mechanically feasible. 

Pattern 

A gunlaying antenna should have low side lobes, 
narrow beam (10-degree maximum) and crossover 
power consistent with the requirements of crossover 
slope and maximum range tracking. Low side lobes 
are desirable to permit search and track beyond alti- 
tude signal range. A narrow beam will permit track- 
ing at low altitudes. The latter consideration is of 
prime importance in the design of night-fighter radar. 
It is important that the main lobe be nearly sym- 
metrical in the planes of the electric and magnetic 
vectors (E and H planes) to prevent the introduction 
of an excessive variable phase shift in the funda- 
mental of the return signal and the introduction of 
excessive harmonics. 22 

Search Scan 

The most commonly used search scan is a type of 
Palmer scan illustrated in Figure 3. Azimuth and 
elevation coverages are limited by mechanical con- 
siderations and by the maximum time interval which 
can be tolerated for a complete scan. Elevation cov- 
erage is further limited by the beamwidth and the 
offset angle. When the offset is obtained by tilting the 
reflector off the rotation axis, the elevation coverage 
for a single azimuth scan is approximately equal to 
the beamwidth plus four times the offset angle. If, as 
in AN/APG-3 and AN/APG-19, the antenna is de- 
signed to have two offset angles, the smaller one for 
track and the larger one for search, increased eleva- 
tion coverage can easily be obtained. 


DESIGN CONSIDERATIONS 


207 


Scanning loss is negligible in this type of search 
pattern, for azimuth and elevation slewing rates are 
limited to fairly low values. 

It is difficult, however, with this type of scan to 
obtain large coverage in the short period required. 
One means of obtaining large coverage and a short 
period is by use of a rotating reflector and feed as- 
sembly nodding from the axis of rotation. The Sperry 
AGL-2 spiral-scan antenna is of this type. It gives 
hemispherical coverage with an effective period of 
1 sec. 

18.3.3 Servomechanism 

Only the high lights of the servo design problem 
will be treated in this chapter. A more complete dis- 
cussion may be found in reference 56a. 


are applied to the grids of each pair of tubes. The dif- 
ferential d-c voltages on the cathodes are a measure 
of the amount and direction of the error. There is no 
differential d-c output for even harmonic signal in- 
put, but there is a differential output for odd har- 
monics. When considerable amounts of second har- 
monic are present in the error signal, as where the 
system has a rotating feed and hence rotating polar- 
ization, this component must be attenuated before 
the signal is commutated. Otherwise the commuta- 
tion interval depends upon the size of the second 
harmonic, and the tubes are not dynamically bal- 
anced. If square-wave commutation for intervals of 
120 degrees is used, the effect of odd harmonics is 
made negligible and the third harmonic is elimi- 
nated. 22 


Error-Signal Filtering and Commutation 

If the beamwidths in the E and H planes of the 
antenna radiation pattern were the same, if they did 
not change with rotation (except, of course, for shift 
of center), and if the r-f polarization remained con- 
stant, then 22 the frequency components of the return 
signal from the target would be so phased that the 
effect of commutation upon each component would 
be to deliver voltages to the scanner servo-amplifier 
which would cause the scanner to move in the proper 
direction. All these conditions are not realized in 
practice; however, they are approached in a well- 
designed antenna. Therefore the third detector net- 
work, the error-signal filters, and the amplifier should 
pass the signal undistorted and with small relative 
phase shift between the fundamental frequency and 
its harmonics, if these harmonics are to be put to use 
in driving the scanner. The band-pass of these net- 
works is chosen so as to reduce or eliminate unde- 
sired components in the error signal. 

Commutation of the error signal can be described 
briefly as follows. A sine wave error signal can be 
broken up by commutation (as illustrated in Fig- 
ure 5) into d-c components corresponding to the 
angular pointing error in azimuth and elevation. 
Current automatic airborne gunlaying systems em- 
ploy full-wave commutation at the conical-scan fre- 
quency. Taking the error-signal channel of the servo- 
amplifier of AN/APG-1 or -2 (illustrated in Figure 6) 
as an example, the operation is as follows. Four sine 
wave voltages phased 90 degrees apart are applied to 
the plates of the commutator tubes in pairs. Each 
tube is then turned on for cycle of the conical-scan 
rotation. The error signals from a push-pull amplifier 


Stability and Time Lag 

When the scanner is mounted on the turret, special 
servo stability problems arise. As shown in Figure 7, 
the radar delivers d-c voltages to drive the turret in 
which the computing sight is mounted. The sight de- 
livers a signal to a servo-amplifier which sets the 
scanner off by the amount of the lead angle. The time 
constant of the sight is variable, increasing with 
range. 58 Synchronization, that is, switching from 
search to track, is done at long range, where the time 
constant of the sight is greatest and where the lead 
angle is greatest. Overshooting is greatest at long 
range, because of the large lead angles; this presents 
a serious problem because of the large time constant 
of the sight. Unless special means are taken to pre- 
vent this condition, the time for synchronization is 
intolerably long. One such means, utilized in one ver- 
sion of AN/APG-1 6, is to set the time constant of the 
sight at a small value and hold it there until syn- 
chronization is almost completed. This is accom- 
plished by fixing the range input to the sight at a 
medium value until the turret is within a degree or 
so of the position computed for that range. At that 
time radar range takes over and synchronization con- 
tinues smoothly and quickly. Further discussion of 
stability is given in Section 19.1.3. 

Search, Pointing, and Tracking Provisions 
The Palmer-scan search function may be accom- 
plished by feeding d-c voltages in the proper sequence 
and duration into the scanner servo-amplifier from 
sources directly on the scanner controlled by cams 
and microswitches. Alternatively, a-c scanning volt- 
ages may come from a separate motor-driven switch- 


RI 


RESTRICTED 


208 


AUTOMATIC FOLLOWING EQUIPMENT 



COLUMN a 


COLUMN b 


PLATE LEADS AS SHOWN 


PLATE LEADS INTERCHANGED 


REFERENCE VOLTAGE 
ON PLATE 2 


SINE COMPONENT OF 
ERROR SIGNALON GRID I 


VOLTAGE AT CATHODE 
DUE TO SINE COMPONENT 
(CONTRIBUTION OF A) 


COSINE COMPONENT OF 
ERROR SIGNAL ON GRID I 


VOLTAGE AT CATHODE 
DUE TO COSINE COMPONENT 
(CONTRIBUTION OF A) 


REFERENCE VOLTAGE 
ON PLATE 5 


SINE COMPONENT OF 
ERROR SIGNALON GRID 4 









VOLTAGE AT CATHODE 
DUE TO SINE COMPONENT 
(CONTRIBUTION OF B) 


COSINE COMPONENT OF 
ERROR SIGNAL OF GRID 4 


VOLTAGE AT CATHODE 
DUE TO COSINE COMPONENT 
(CONTRIBUTION OF B) 


VOLTAGE AT CATHODE DUE 
TO SUM OF CONTRIBUTIONS 
OF TRIODES A AND B 



Figure 5. Basic unit of phase detector — functional diagram. 


RESTRICTED 


DESIGN CONSIDERATIONS 


209 


AZ TRACK AZ D-C 

PHASE DETECTOR POWER AMPLIFIER AZ ANTI-HUNT CONTROL 



Figure 6. Simplified schematic of AN/APG-1 and -2 servo-amplifier circuits for automatic tracking. 


ing source, in which case there are selsyn tie-ins with 
the scanner, and the selsyn signals are delivered to 
the scanner servo-amplifier. 

If the scanner is mounted on a stationary platform, 
pointing or searchlighting may be accomplished by 
feeding the output signals of selsyn or potentiometer 
linkages between the sight or hand-sight control and 
the scanner into the scanner servo-amplifier. If the 
scanner is mounted on the turret, the same may be 
accomplished, of course, by positioning the turret. 

Tracking is accomplished by feeding the radar 
error signal into the scanner servo-amplifier or, if the 
scanner is mounted on the turret, by feeding the d-c 
outputs of the commutator into the turret servo- 
amplifier. 


18.3.4 Automatic Range Tracking 

For detailed information on particular range cir- 
cuits, consult the Radiation Laboratory Technical 
series, especially reference 54. 

Aside from meeting the accuracy requirements set 
forth in Section 18.2, it is desirable to have the range- 
circuit design provide for operation against window 
countermeasures. If a night-fighter AGL range cir- 
cuit tracks on the trailing edge of the echo, and if the 
range circuit of the defensive type of equipment 
tracks on the leading edge of the echo, window is 
much less likely to pull the range gate off the target. 
This is because the range motion of the window is in 
a direction opposite to the direction of the unbalance 
of the tracking gates. Such unbalance may be 


SIGHT 



AZIMUTH AND 
ELEVATION 
DEFLECTION 
SCANNER SERVO 
AMPLIFIER 





Figure 7. 


Block diagram of typical servo system when antenna is mounted on the turret. 



ST RIOTED 







210 


AUTOMATIC FOLLOWING EQUIPMENT 


achieved by providing unequal gains in the dual gate 
networks used for automatic range tracking. 

Defensive systems require automatic range search 
and lock-on with provision for advancing the range 
gate to adjacent targets. The automatic range search 
limit should be adjustable to keep it within the alti- 
tude signal. 

18.3.5 Automatic Frequency 

Control 

Asymmetry in the r-f joints, faulty contacts, and 
nonuniformity of antenna housing can cause fre- 
quency pulling of the magnetron. Frequency pulling 
at the conical-scan rate and its harmonics can cause 
mispointing of the antenna, the degree of mispointing 
depending upon the amount and character of the fre- 
quency pulling, the band-pass characteristic of the 
receiver strip, and the amount of local oscillator de- 
tuning. A fast-acting AFC will eliminate most of such 
trouble. The required AFC characteristics are de- 
termined by the nature and amount of pulling in- 
volved, the time interval during which the pulling 
takes place, and by the amount of pulling which could 
be tolerated without AFC. The amount which can be 
tolerated can be calculated from knowledge of the an- 
tenna pattern, the i-f band-pass characteristic, 47 and 
the loop gain of the servo. 

18.3.6 Automatic Gain Control 

Automatic gain control [AGC] is required to main- 
tain the signal level at the receiver output as the 
strength of the received signal varies because of fad- 
ing and range changes, and to hold the effective 
servo-loop gain constant. It must be sharply attenu- 
ated at the error-signal frequency to prevent loss of 
and phase shift in the error signal. 


eter position voltage. Its purpose is to indicate the 
instantaneous azimuth position of the main lobe. 

When the system is tracking, the range track gate 
is also shown on the scope in one of several ways. It 
may be in the form of a video signal similar to the re- 
turn echo when on target (AN/APG-1, -2, and -3), 
or as a dot displaced in azimuth from the target echo 
(AN/APG-16). If the scanner is mounted on the tur- 
ret, a turret position marker may be shown periodi- 
cally as well. A type C display (elevation versus 
azimuth) is made available in night-fighter systems 
so that the operator can tell the pilot the elevation of 
the target. 

Night-fighter systems require a modified type G 
presentation on a remote scope for the pilot, so that 
he can home on a target and follow a maneuvering or 
jinking target. In addition to the spot and wings in- 
dicating the angular position and range of the target, 
which the type G presentation provides, an artificial 
horizon is put on the tube. Mechanical or electronic 
switches are used to alternate these indications on the 
tube face so that there is no discernible flicker. En- 
graved lines on the tube face cap indicate firing range. 

Spiral-scanning systems require a combination of 
two or more indicators. One combination, for exam- 
ple, is a B scope and a B' (elevation versus range) 
display. 

Meters 

Night-fighter AGL radars must indicate range and 
range rate to the pilot. Meters can perform this func- 
tion and they place little or no burden on the range 
unit. 

In some systems jitter meters have been provided. 
These indicate the amplitude of the jitter of the scan- 
ner around the target. Such a meter is useful if the 
servo audio gain control is available both to the oper- 
ator and for servicing purposes. 


18.3.7 Indicators 

Scope Presentation 

The B type of presentation (range versus azimuth), 
to which has been added a small a-c voltage on the 
horizontal plates (see Figures 8 and 9), is the basic 
indication used on search and track in all production 
AGL systems designed to date (1945) in the United 
States. The a-c voltage is at the same frequency and 
in phase with the azimuth component of the conical- 
scan motion and is added to the azimuth potentiom- 


18.3.8 Antijamming Considerations 

A detailed discussion of antijamming provisions is 
beyond the scope of the present chapter. In general, 
however, it may be said that antijamming precau- 
tions, such as narrow beam, short pulse duration, 
wavelength flexibility, short time constant following 
the video detector, instantaneous automatic volume 
control, unbalanced tracking gates, blanking the 
initial pulse in the video amplifier rather than the 
i-f strip, and vertical antenna polarization are all ap- 
plicable to automatic gunlaying systems. 




DESIGN CONSIDERATIONS 


211 



Figure 8. Typical presentation of B scope when 
system is searching. 



RANGE I 
•♦MARKERS 


AZIMUTH SCALE 


TURRET 

POSITION 

MARKER 

O 


AZIMUTH SCALE 


A 


B SCAN DISPLAY OF 
AN/APG - I AND 
AN/APG - 2 


C SCAN DISPLAY OF 
AN/APG - 16 


Figure 9. Typical presentation when systems are tracking. 


18.3.9 Special Provisions 

Boresighting 

The scanner radar boresight direction and the gun 

\ 


boresight line must be harmonized, and provisions 
must be made in the design of the radar to permit the 
determination of the radar boresight direction, and to 
facilitate the optical harmonization . 30 ’ 32 


RESTRICTED 








212 


AUTOMATIC FOLLOWING EQUIPMENT 


When there is a second harmonic in transmitted 
power along the radar boresight line because of asym- 
metry of the beam in the E and H planes, a simple 
first echelon procedure for determining the line can 
be set up. 36a In general boresighting presents serious 
problems which are not treated in this book. 

IFF 

Identification is highly important and usually 
not easily accomplished. In many cases, IFF equip- 
ment requires that the radar supply a synchronizing 
trigger and accept a video response. Some systems 
such as Black Maria place special design require- 
ments on the radar. This, and other IFF equipment, 
is discussed in the Radiation Laboratory Technical 
Series. 57b 

Warning Buzzer and Light 

Defensive AGL equipment is used on long tedious 
flights in which the gunner is likely to lose his alert- 
ness. For this reason it is desirable to have the radar 
automatically give visual and aural notice in the form 
of a light and a buzzer or similar device operated by 
the range circuit when a target is detected in the 
search area. 

18.4 ELEMENTS OF PARTICULAR SYSTEMS 

Figures 10, 11, 12, 13, and 14 illustrate various 
types of AGL antenna assemblies and designs. Most 
of them are units of production systems. 

18.5 ACCURACY 

18.5.1 Photo Scoring 

Tracking tests were made at East Boston Airport 
and Boca Raton, Florida, on an AN/APG-1 system 
installed in a YP-61 night fighter, using small air- 
craft such as an AT-11 or an 0-47 as targets. Analysis 
of these films showed a probable error in antenna 
tracking of 2 to 4 mils in both azimuth and elevation. 
Greater errors, when they occurred, were found to be 
caused by malfunctioning of some part of the equip- 
ment. Evaluation of the films at close range is diffi- 
cult because the target subtends a considerable angle. 
The films at close ranges were analyzed on the basis 
of hits in vital areas compared with the total number 
of rounds. Data were taken from every fifth frame of 
films made with an exposure film speed of 16 frames 
per sec. 

Film records indicate that there may be some spe- 
cial computer or possibly radar servo problems aris- 


ing in connection with night-fighter AGL. The usual 
night-fighter target is a medium or heavy bomber, 
and it is found that the radar beam will wander over 
the target airplane when tracking, although favoring 
the engines somewhat. This wandering occurs at 
rates which can be interpreted by the computer as 
target motion and may, therefore, result in false gun 
aim. Further study of this problem is needed. 

18.5.2 Firing Tests 

Tests of AN/APG-1 installed in a YP-61 equipped 
with a modified General Electric 2CH1L2 Central 
Station computer were conducted at Brownsville, 
Texas. A rectangular plastic flag, 6 X 30 ft, with a 
centered corner reflector was found to be the best 
target. 137 

Some of the results (using this type of target) are 
given in Tables 1 and 2. They compare very well with 


Table 1 . Sitting duck missions. 


Range 

Target position 

Rounds 

Hits 

Score 

300 yd 

45° az., 20° el. 

400 

160 

40% 

900 yd 

45° az., 20° el. 

400 

22 

5.5% 

900 yd 

45° az., 20° el. 

400 

14 

3.5% 


Table 2. Passing attack missions. 


Range 

Target position 

Rounds 

Hits 

Score 

400-300 yd 

35° to 75° az., 20° el. 

400 

160 

40% 

400-300 yd 

35° to 75° az., 20° el. 

400 

136 

34% 

650-500 yd 

35° to 75° az., 20° el. 

400 

19 

5% 


the scores of a good gunner firing under ideal condi- 
tions without radar aid. However, AGL cannot yet 
replace the gunner. It is a complex piece of equip- 
ment and, at least at this stage of the art, cannot be 
expected to be operational 100 per cent of the time. 
It is to be hoped that, in the future, completely auto- 
matic operation can be used at all times, and provi- 
sions made for the gunner to take over only if the 
system fails. This would be an advance over the pol- 
icy prevailing in World War II, namely, that in day- 
time missions the radar should provide range only. 

18.6 SOME POSSIBILITIES FOR 
FUTURE EQUIPMENT 

Some of the following suggestions have been tested 
in Service equipment, in which case reference is made 
to the Army-Navy designation of the equipment; 
others are ideas on paper only. 


SOME POSSIBILITIES FOR FUTURE EQUIPMENT 


213 


18.6.1 Components 

Antenna 

The necessity for large search and track coverage 
for both offensive and defensive AGL systems has 
been discussed in Section 18.2. All antennas designed 
to date have had to perform both of these functions, 
and compromises have been made in size, weight, 
search and track coverages, or period. Future com- 
promises of this sort may endanger the value of the 
equipment. While it may not be impossible to design 
a single ideal antenna, it is certainly much more obvi- 
ously possible to design two antennas, one for search 
and one for track, which will meet all performance 
requirements. Antennas designed at K band might 
come close to meeting the size and weight require- 
ments as well. For the track antenna a rotating dish 
design would be best at this short wavelength, and 
for the same performance the dish need be only 
about one third the diameter of the corresponding 
X band antenna. Consequently, the conical-scan 
motor could be less powerful and therefore lighter. 
The azimuth-elevation drive motors could be smaller 
because the antenna need no longer search, and their 
operation would be infrequent and of short duration. 
Furthermore, if the antenna were mounted on a tur- 
ret and therefore need only be capable of relative 
angular displacement equal to the lead angle, an ad- 
ditional reduction in size and weight could be made 



Figure 10A. AN/APG-1 antenna and transmission 
line (schematic). 


from that of a corresponding antenna which must 
search as well as track. The track antenna for night- 
fighter work would, of course, use a larger reflector to 


obtain a narrow beam for low altitude work than 
would be required for the track antenna for a bomber 
defensive system. 



Figure 10B. AN/APG-1 antenna. 

The search antenna must have a beam which is 
fairly narrow in the E and H planes to provide angu- 
lar discrimination on airborne targets. It would be 
desirable to design the search antenna so that it 
would permit indications suitable for navigation and 
bombing as well as for aerial search. Assuming that 
hemispherical scan is desired of the search antenna, 
the approach to the problem may be made in two 
ways. An antenna may be patterned after an existing 
one such as the AIA spiral scan antenna (see Chap- 
ter 15), in which case the nod angle would be in- 
creased to ±90 degrees. The second approach is to 
design an entirely new type of antenna. Figure 15 
illustrates this. The antenna and housing comprise 
one integral unit. They rotate at fairly high speed in 
the slant plane (axis A); the slant plane is rotated 
slowly in comparison (axis B ) . The maximum ratio of 
high speed rotation to low speed rotation is equal to 
360 degrees divided by the beamwidth (in degrees). 
The antenna housing contains, in addition, the high- 
speed motor drive and gear box and the necessary 



214 


AUTOMATIC FOLLOWING EQUIPMENT 



Figure 1 1 . AGL-2 antenna. 


potentiometers for delivering position information to 
the indicator. This antenna could be installed either 
outside or inside the airplane. Periods of about 1 sec 
could be achieved with both antennas, although with 
hemispherical coverage very short periods are not 
necessary. 

Rapid television-scan antennas 20 ’ 55a operating at 
wavelengths of the order of 0.6 cm have been pro- 
posed for use as AGL tracking antennas in offensive 
systems. While the use of such antennas would pre- 


sent special angular track circuit problems, such 
problems are not insurmountable. With such anten- 
nas it would be possible, if the target were a heavy 
bomber, to present the airplane silhouette for identi- 
fication purposes at short ranges, and to forewarn the 
operator or pilot of evasive action by the target. 

Indication 

The two search antennas discussed above present 
special indicator problems. If a normal type B indi- 


SOME POSSIBILITIES FOR FUTURE EQUIPMENT 


215 


cator is used, a large part of the search volume in the 
lower part of the hemisphere (assuming the center of 
coverage to be horizontal) will be lost because of 
ground echoes and screen persistence. This undesir- 
able condition might be circumvented by use of a 
variable range gate generated in such a fashion as to 
blank out the indicator short of the range to ground 
for the instantaneous positions of the antenna, as il- 
lustrated in Figure 16. The B scope would then indi- 
cate all targets above a certain altitude only, except 
for strong off-axis ground targets. For a search an- 
tenna which rotates in the slant plane, the range to 
the altitude below which the indicator is blanked is 
given by (h — a) /cos a cos 0, where (90 — a) degrees is 
the depression angle of the slant plane, the angle 
of search in the slant plane, h the altitude of the air- 
plane, and a the search altitude. The generation of a 
d-c voltage proportional to this quantity to be de- 
livered to the indicator sweep generator for the pur- 
pose of obtaining a blanking gate would be a rela- 
tively simple matter. It would be necessary to shorten 
this range gate for large depression angles because of 
increased signal return from ground targets off the 
axis of symmetry of the main lobe, unless the use of 
expanded gain were to make such a device unneces- 
sary. A type C indication would be a useful adjunct, 
if the variable range-blanking gate and expanded 
gain were incorporated into the system. 



Figure 12. AN/APG-3 antenna. 


Expanded Gain 

Expanded gain, used in AN/APG-13B (see Sec- 
tion 20.5), is the addition of a small portion of the 
indicator sweep voltage to the receiver gain voltage. 
It results in a more uniform indication of target 


brightness on the B scope, regardless of range. Re- 
ceiver gain is small at short ranges and full on at 
maximum ranges. Its use with AGL has the following 
advantages : 

1. AGC action need not be so great. 



Figure 13. AN/APG-16 antenna mounted on turret. 


2. There is more uniform target brightness on 
B scope presentation. 

3. It makes possible the use of type C indication, 
where desirable, by eliminating one of the chief ob- 
jections to the latter, namely, the inherent low ratio 
of target brightness to background brightness caused 
by the piling up of noise. 

4. Its use simplifies the problem of gating out 
ground echoes on the type B search indication as 
described in “Indication/’ Section 18.6.1. 

Automatic Range Search and Lock-on 

A modification of circuit design developed for 
AN/APG-21, Terry (Section 20.6), which enables it 
to search for and lock on unmodulated signals only, 
would be a useful adjunct to the daytime use of AGL 
equipment. The modification would consist of per- 
mitting the range gate to lock on a slightly modulated 
echo and therefore not be thrown off if the operator’s 
tracking is erratic. Its use would eliminate one of the 
present duties of the operator, namely control of the 
range switch. 


216 


AUTOMATIC FOLLOWING EQUIPMENT 


REFERENCE VOLTAGE e, 
FROM TWO -PHASE GENERATOR 



Figure 14. Block diagram of|AN/APG-l antenna servo amplifier. 


WAVE GUIDE 



Figure 15. Simplified sketch of hemispherical search antenna assembly. 

RESTRICTED 1 






SOME POSSIBILITIES FOR FUTURE EQUIPMENT 


217 


ANTENNA 





Figure 16. Avoidance of ground signals by range 
gating. Indicator is blanked for signals coming from 
targets whose altitudes are less than a. (The target air- 
plane in the figure is at the lowest altitude for which 
detection would be possible.) 

18.6.2 Systems 

Defensive Systems 

The use of separate antennas for the search and 
t rack functions would provide the basis for a central- 
ized radar fire-control system for bomber protection. 
Two hemispherical search antennas, one fore and one 
aft, would give a fire-control director the range, bear- 


ing, and heading of all airborne targets within the 
operating radius of the equipment, assuming that 
variable range-blanking gate and expanded gain (see 
“Indication” and “Expanded Gain” inSection 18.6.1) 
are incorporated in the equipment. Tracking anten- 
nas placed on each turret, driven by signals from 
computing sights contained therein or by a central 
station computer, would provide automatic tracking 
and deflection shooting. 

It might not be necessary to have a complete radar 
set for each of these tracking antennas, if pairs of 
turrets were not too far separated. In that event a 
single r-f head could feed both antennas by having 
the fire-control director switch the power to the de- 
sired one. Indicators would be centralized and would 
not be needed at the local stations. Uninhabited tur- 
rets could be remotely positioned and fired by the 
fire-control director. Target position information 
could be transmitted to inhabited turrets in any one 
of a number of standard methods. Video signals from 
local systems, channeled to the central indicator, 
would give the fire-control director a constant check 
on the operation of local systems. Triggering all mod- 
ulators at the same time would reduce duplication in 
indicator circuits, for then two sweep generators, one 



Figure 17. 


Central radar control for a possible bomber fire-control system. 


RESTRICTED 


m 



218 


AUTOMATIC FOLLOWING EQUIPMENT 


short-range, one long-range, could supply all indi- 
cator tubes. Figure 17 illustrates the control of a 
hypothetical armament system. 

Offensive Systems 

Simplifications in AGL equipment since the design 
of AN/APG-1 and AN/APG-2 make it entirely pos- 
sible that a night-fighter pilot could operate it. The 
pilot would have a remote scope, a control switch, 
and possibly a target in-out range switch. 

With slight modifications in the circuits, a night- 
fighter AGL system could be made to perform the 
daytime ground support functions of air-to-ground 
and air-to-sea attack performed by AN/APG-2 1 (see 
Chapter 20) . The night fighter would then become a 
very potent day fighter as well ; the system would be- 
come a universal AGL radar. Briefly, the modifica- 
tions required are as follows. 

1. The addition of rejection circuits in the range 
unit to permit locking in range on unmodulated sig- 
nals only, thereby causing the range unit to track the 
target toward which the pilot is aiming his sight. 

2. Tie-in with a computer or a computing sight, 
such as the Draper-Da vis S-9, delivering the proper 
depression angle information to drive the scanner. 
The error signal would not be used in these opera- 
tions. 


By enabling the pilot to switch his indication to 
one in which the radar error signal is presented as a 
spot deflection or its equivalent, these operations 
against sea targets might well be performed at night. 
Where it is necessary to pick a particular target out 
of several, it might be necessary to introduce the 
AN/APG-13B type V presentation, conical-scan 
angle versus range (see Chapter 20), to permit the 
pilot to line up on his target accurately before 
switching to track and starting his run. 

Tests have been made by the Armament Division 
at Wright Field on SCR-702-T1 tied in with auto- 
matic flight control equipment [AFCE]. The equip- 
ment was installed in the XA-26A airplane. The 
AGL equipment caused the XA-26A to follow the 
target automatically. These tests are significant 
when one considers that the night fighters’ prey of 
the future would in all likelihood be an ultra-fast jet- 
propelled airplane or a guided missile wherein split- 
second timing of a degree beyond human capabilities 
might be required to make successful kills. It would 
be possible to tie in automatic-following equipment 
and computers with AFCE to make a completely 
automatic interception and to fire automatically or 
launch with the proper deflection angle the lethal 
missile — be it bullet, shell, proximity-fused bomb 
or rocket, or the airplane itself. 


a 


RESTRICTED 


Chapter 19 

MANUALLY DIRECTED RADAR GUNSIGHTS 


19.1 GENERAL CONSIDERATIONS form as developed in World War II is less accurate 

than AGL. 


19.1.1 Type of Application 

Some military requirements for fire-control radar 
cannot be met at the present state of radar develop- 
ment with the fully automatic gunlaying equipment 
[AGL] discussed in Chapter 18. Such requirements 
take the form of restrictions of weight, volume, and 
complexity. If reduced performance is tolerable, the 
restrictions can often be met by a manually directed 
radar gunsight. The general name for any radar sight 
of this class is airborne gunsight [AGS] ; the particular 
system discussed below is AN/APG-15 35 (see Sec- 
tion 19.2). In general, such a system finds application 
only when the more completely automatic equipment 
would be too large, too heavy, or too complex. The 
following examples of these conditions can be given. 


Heavy Bombers 

As an adjunct to a fire-control system designed for 
optical tracking, a manually operated radar gunsight 
is more feasible than a complete AGL system. Usu- 
ally the installation of an AGL system requires sev- 
eral modifications to existing bombers, whereas a 
manually directed radar gunsight requires few, if 
any, modifications. For little more than the cost of 
adding radar ranging, a radar gunsight can be ob- 
tained. However, the decision as to the type of equip- 
ment to be used is definitely a compromise between 
accuracy on the one hand and weight, bulk, and com- 
plexity on the other. Manually directed radar gun- 
sights found most applications in tail defense of 
heavy bombers, in theaters where enemy night fight- 
ers were not very aggressive. 


Shipborne Night Fighters 

In general, the number of types of aircraft used on 
carriers is kept to a minimum and, as a result, it is 
highly desirable to use standard single-place fighters 
for both day and night fighting (see Chapter 14). In 
carrier fighters, therefore, it is impossible to use radar 
equipment that requires a special operator; likewise 
it is important that the equipment be small and 
lightweight. As a consequence of these considerations 
the night-fighter radar for carrier-based fighters is of 
the manually directed type, that is, the pilot uses his 
aircraft as a gun mount and acts as a biomechanical 
servomechanism between the radar indication and 
the guns. 

Land-Based Night Fighters 

An airplane like the P-61 with an upper remote 
turret is admirably suited to completely automatic 
gunlaying. However, if the plane has aircraft inter- 
ception [AI] equipment like the SCR-720, an auxil- 
iary radar is needed to make the turret effective. 
Such a radar could be of the manually directed type, 
since neither long range nor search is a requirement. 
This type of combined installation is somewhat more 
flexible than a straight AGL installation, allowing 
continuous long-range search while the manually 
operated radar handles near-by targets. However, 
the disadvantage of such an installation is that in 
addition to requiring an operator, the lightweight 


19.1.2 Methods of Achieving 
Simplicity 

Component Packaging 

One of the most direct methods of achieving sim- 
plicity is to use small, individually cased components. 
For example, the lighthouse tube transmitter and re- 
ceiver [LHTR] (see Section 20.2) is admirably suited 
to such systems. The LHTR contains the modulator 
and r-f components of the transmitter, r-f compo- 
nents and i-f strip of the receiver, and power sup- 
plies. This packaging facilitates installation and 
maintenance (see Section 5.1). Also, the ranging or 
timing components, or the indicating components 
can be packaged together. By such functional pack- 
aging the system itself is readily adapted to different 
installations. By standardizing the functions of such 
units an additional advantage is gained in that each 
component can be designed to give the desired per- 
formance of its function, that is, one transmitter- 
receiver design can be substituted for another if 
greater range, different wavelength, etc. are desired; 
or one range unit may be substituted for another if, 
for example, different range accuracy or range sweep 
is desired. 

Type of Indication 

In almost all radar sets it is necessary to convert 
the electrical radar information to an optical presen- 



219 


220 


MANUALLY DIRECTED RADAR GUNSIGHTS 


tation. Such conversion components, called indi- 
cators, are often very complex and consequently re- 
quire considerable power and space. To simplify an 
indicator it is necessary to sacrifice some desirable 
qualities such as low distortion or multiple target 
presentation. For instance, in the AIA equipment 
the indicator allows a choice of B or G type, the B 
presentation (range versus azimuth) being used for 
search and the G presentation (see Section 15.1.9) for 
gun pointing. In the AGS equipment the indicator is 
type G. In both the AIA and AGS systems the 
amount of information presented on the indicator is 
a compromise between simplicity and component 
size. 

Type of Antenna 

The type of indication used is closely allied to the 
antenna design and simplified indicators probably 
save more in the antenna than in the indicator com- 
ponents. Antenna design is extremely important, 
especially when the antenna must be mounted 
externally and thereby contributes to drag. The 
mounting of a large antenna may also require changes 
in the design of the aircraft, -so it is generally worth 
while to keep the antenna small and the mounting 
arrangements simple. 

Perhaps the best illustration of this design princi- 
ple is the AGS antenna used in the AN/APG-15 
system. 35 As in all current airborne gunlaying sys- 
tems, the antenna performs a conical scan (Figure 2, 
Chapter 18); for simplicity this system employs a 
rotating paraboloid with a fixed feed. The phase ref- 
erence generator is coaxial with the rotating member 
and is essentially an electric switch. The whole an- 
tenna assembly is a ball-like package, 16 in. in diam- 
eter and weighing 20 lb. Mounting is accomplished 
by connection to four existing bolts in the B-29 tail 
turret. In this design high pointing accuracy is sacri- 
ficed for greater coverage; since there is no inde- 
pendent search scan the beamwidth and conical 
scanning must be broad enough to give sufficient 
coverage while allowing reasonable pointing accur- 
acy. 

Type of Antenna Mounting 

As indicated in the paragraph above for the AN/- 
APG-15 case, considerable complexity is eliminated 
by mounting the antenna directly to the guns. This 
is readily accomplished for a small unit such as is 
used in the AGS or airborne range only [ARO] sys- 
tems. Obviously this mounting arrangement is not 


always possible; but, where possible, it greatly sim- 
plifies installation of the equipment. 

19.1.3 Methods of Achieving Simple 
Computer Tie-Ins 

The use of manually directed radar systems in con- 
j unction with lead-computing mechanisms has not 
found general application. However, such systems 
have been studied and some experimental installa- 
tions made. Computer tie-ins fall into two distinct 
classifications : systems in which only the radar range 
information is fed to the computer, and systems 
which permit computed blind fire. Figure 1 shows a 
block diagram of these two classifications as well as a 
basic radar system with no computer tie-in. These 
diagrams show the fundamentals irrespective of type 
of installation or computer; obviously the mecha- 
nization of such systems is dependent upon both. 

Sights for Fixed Guns 

For fixed-gun fighters the second type of computer 
tie-in is made, if any tie-in is made at all. The prob- 
lems of flying an airplane by means of a spot error 
indication (or any other angular high-precision indi- 
cation) are so severe that blind attacks are, in gen- 
eral, limited to straight tail attacks. For such attacks 
no deflection is necessary and, consequently, no com- 
puter is necessary. For daylight operation, however, 
radar range is far superior to optical range, and the 
use of radar range greatly enhances the effectiveness 
of the fighter. 

Director-Type Computer 

In aircraft with fire-control systems like those of 
the P-61 or B-29 the computers are of the director 
type, that is, target position and rate are measured 
independently of the gun position and rate (see Sec- 
tion 21.3.1). It is reasonably straightforward to con- 
nect a manually directed radar set into this type of 
gunlaying system. By mounting the antenna directly 
to the guns, but allowing a few degrees (approxi- 
mately 15 degrees for caliber 0.50 guns) of inde- 
pendent motion, the radar axis can be offset from the 
gun direction by the amount of the lead angle. Such 
an arrangement is efficient since it requires only 
small slow-speed servomechanisms to position the 
antenna with respect to the guns, gross positioning 
being done by the large turret servos. Radar range is 
fed into the computer. 


GENERAL CONSIDERATIONS 


221 



MOUNTED 

A ON GUNS 

POftfr- BLANK RADAR INDICATION 



POINT-BLANK RADAR INDICATION 
RADAR RANGE FOR OPTICAL TRACKING 


COMPUTED POSITION RADAR INDICATION 
RADAR RANGE FOR OPTICAL TRACKING 



I 

GUNS 


Figure 1 . Block diagrams of manually directed gunsights. 


Disturbed-Reticle Sights 

Effecting a complete tie-in with a computer like 
those commonly used in inhabited turrets is fairly dif- 
ficult. Suppose that the radar antenna is mounted rig- 
idly on the turret and boresighted with the gun bore 
axis and that the scope presentation is type G. Then 
the target signal is displaced from the center of the 
G scope by an amount X corresponding to the angle L r 
between the gun bore axis and the line of sight to the 
target. The displacement X is ordinarily not a linear 
function of L', although for angles L' less than 4 de- 
grees a satisfactory linear approximation to X can be 
made. Suppose, next, that the sight case is mounted 
on the guns so that the computer measures angular 
rates of the gun line. Then, to effect a complete tie-in 
of radar and computer it is necessary to set up a re- 
lation between X and the computed lead L such that 
L = L'. For instance, in the British AGL-T the radar 
presentation was superimposed on the sight reticle 


and the matching of L to L' consisted of positioning 
the guns so that the radar target-pip coincided with 
the center pip of the sight reticle. This tie-in intro- 
duced problems of operational stability which are dis- 
cussed below. Aside from stability problems the 
sighting system had errors whenever the lead L was 
sizable, as a result of both the nonlinearity of the re- 
lation between the scope target-pip deflection X and 
the actual lead L', and the variability (static insta- 
bility) of the scope sensitivity. 

The British Gyro Gunsight Mk II-c (upon which 
the U. S. Navy Mk 18, 21, 23, and the U. S. Army 
K-14, -15, -17 were based) was the sight member of 
the AGL-T combination. In this sight (and its deriv- 
atives) the lead is computed optically and there is no 
means provided for delivering a mechanical or elec- 
trical lead signal. For those computers from which a 
mechanical or electrical lead signal output can be 
obtained, it is possible to provide a servo-driven an- 


RESTRICTED 



222 


MANUALLY DIRECTED RADAR GUNSIGHTS 


tenna which can be offset from the gun axis by the 
amount of the computed lead (just as in director 
systems). This overcomes any difficulties caused by 
the nonlinearity of scope deflection and by the vari- 
ation in scope sensitivity, but introduces servo prob- 
lems. These servo problems are not difficult to solve, 
but the related engineering problems were suffi- 
ciently complex that it was not considered worth 
while to attempt their solution during World War II. 
Instead, attention was directed to the more immedi- 
ately available point-blank system described in 
Section 19.2. 

Stability Problems 

As indicated above, the use of a manually directed 
radar gunsight with a disturbed-reticle lead-comput- 
ing sight presents problems of operational stability. 
A disturbed-reticle sight is said to be operationally 
stable if, when the gun is given a small but quick jerk 
(by a jerk of the gun we mean a discontinuity in 
velocity) in some direction the reticle is jerked in the 
same direction; if the reticle is jerked in the opposite 
direction the sight is said to be unstable. 58 ’ 89 

The AGL-T was the only system of this composite 
type used in World War II, and as originally installed 
it proved to be operationally unstable, in spite of the 
fact that each component was stable in itself. The fol- 
lowing discussion of the problem is based upon a re- 
port by Dr. C. W. Gilbert of the British Gunner}^ 
Research Unit. 139 The problem is considered first in 
mathematical terms. 

Consider a fixed direction in space, and let y, p, o-, 
and r be the angles made with it by the gun line, line 
to apparent radar target, sight line (line to moving 
reticle spot), and line to the actual target, respec- 
tively. The task of the gunner is to keep the radar 
and reticle spots, which he sees, in coincidence (the 
gunner is in total darkness, otherwise). This has the 
effect of keeping p equal to <r. It will be noted that 
the computed lead L [neglecting the ballistic term 0 
of equation (1), Chapter 23] is 7 — cr; the lead actu- 
ally being taken by the guns, L', is 7 — r; and the de- 
flection of the radar scope spot X is 7 — p. 

The differential equation for the lead computing 
sight 58 may be written as 

(1 — a) u (7 — ff) + (7 — a) = uy (1) 

where u is the time of flight and a is the sight param- 
eter. (Dots indicate time derivatives.) This is 
equivalent to equation (1), Section 21.3.1, provided 
the ballistic term 0 in the latter is neglected. Solution 



of this equation shows that the sight reticle comes to 
rest, after a sudden motion, with the time constant 
(1 — a)u (see Section 22.2.3, “Errors in Optical 
Range Determination”) provided that a < 1. (For 
0 < a ^ 1, the sight would be operationally un- 
stable; for a < 0 the sight is operationally stable.) 

For the radar target-pip, the differential equation 

k ( 7— p) + (7 — p) = 7 - r (2) 

will apply, where k (> 0) is the time constant for the 
damping of the motion of the spot. 

Now we inquire how it happens that the combined 
system can be unstable when both sight and radar 
are stable (a negative and k positive) . If it is assumed 
that the operator tracks perfectly, that is, keeps the 
reticle pip on the radar pip, then a = p. To test for 
stability of the combined system, set p = a and then 
eliminate <j from the two differential equations, ob- 
taining an equation relating the gun position 7 and 
the target position r. If this equation is that for a 
damped motion, then the gun will follow the target 
and the system will be stable. 

— kuy + ( — a) uy + 7 = (1 — a) ut + r. (3) 

This equation will represent damped motion only 
if the roots of the quadratic equation 

— kux 2 + ( — a)ux +1 = 0 (4) 

have negative real parts, which requires that the co- 
efficients of x 2 and x both be positive. Since k is al- 
ways positive, the coefficient of x 2 must be negative, 
and therefore the system is operationally unstable. 

In physical terms this means that if there is any 
optically appreciable lag in the motion of the radar 
spot, the gunner will have to move the turret in the 
opposite direction from that of the target motion in 
order to make the two spots come together; but when 
he has done this, the spots will immediately drift 
apart again, and his only chance of tracking is to 
make a series of trial motions, waiting for equilibrium 
each time. This would obviously be impossible for a 
target with a high angular rate. 

The instability could be removed by either (1) 
greatly reducing the time lag k of the radar or (2) in- 
creasing a so that the sight line would follow the gun 
line more closely, or (3) combinations of (1) and (2). 
In practice, neither (1) nor (2) was feasible. The solu- 
tion adopted was to feed onto the display, in series 
with the radar error voltages, additional voltages 
proportional to the acceleration of the guns in ele- 
vation and of the turret in azimuth. 88a> 139-141 


SYSTEM AN/APG-15B 


223 


19.1.4 Identification 

One of the most serious problems in the design of 
any gunlaying set is identification of a target as friend 
or foe. The problem is equally serious whether the 
gunlaying set is designed for defensive installations 
in bombers or offensive installations in fighters. How- 
ever, the problem does have different aspects for the 
two applications. In general, the burden of identifi- 
cation rests with the attacking aircraft, so it is rather 
more important for the fighter to identify its target 
correctly than for the bomber to identify its target 
correctly. This is based on the assumption that if a 
fighter attacks a friendly bomber, the net loss is 
smaller if the fighter is shot down than if the bomber 
is shot down. From the fighter's point of view, the 
identification equipment must be reliable and secure. 
Reliability can be obtained by proper engineering 
design; but security depends more on flexibility of 
design and inherent design characteristics. 

Identification without Special Equipment 

Under certain operational conditions it may be 
possible to accomplish identification without the use 
of special equipment. Such identification is usually 
accomplished by the employment of special tactics 
or procedures. For instance, this system is applicable 
to the identification of enemy night fighters by un- 
escorted bombers. If the bombardment aircraft con- 
tains manually directed fire-control equipment, or 
automatic fire-control equipment, and can determine 
when an aircraft is trailing the bomber, it is generally 
sufficient to establish that the detected aircraft is 
trailing and is not in the line of fire by mere chance. 
This can be accomplished by an abrupt change of 
course. If the detected aircraft follows this maneuver, 
it can reasonably well be assumed to be a radar- 
equipped enemy night fighter; whereas if the detected 
aircraft does not follow this maneuver it can reason- 
ably well be suspected of being in the cone of fire by 
chance. As mentioned in the paragraph above, this 
means of identification is of practical value only in 
theaters where the burden of identification rests with 
the attacking airplane. 

Beacon Responders 

In most cases it is necessary to establish general, 
bilateral identification, and to this end, beacon chal- 
lengers and beacon transpondors have been developed 
to a high degree of perfection. 57 In these systems the 
identification equipment is a separate and distinct 


electronic component and interconnects with the 
fire-control equipment only to the extent necessary 
to give target identification. A discussion of the tech- 
nical features of the beacon systems used for identi- 
fication is not within the scope of this chapter; such 
systems do exist and work satisfactorily. The biggest 
single objection to this type of identification is in its 
inflexibility. Since, in any given theater, it is neces- 
sary to install a very large number of these systems, 
it is very difficult to change if the security is ques- 
tioned. As far as the parent radar system is concerned 
it is reasonably easy to standardize input and output 
for the identification unit, such standardization per- 
mitting different types of beacon units to be used 
without modification to the parent radar sets. 

Propeller Modulation 

An effective system of identification can be instru- 
mented on the basis of the propeller modulation of 
the detected aircraft. A system utilizing this princi- 
ple, the AN/APX-15, was designed and used. This 
system is particularly applicable to the fire-control 
radars because they are, in general, designed to per- 
mit careful modulation studies of each individual 
echo return. Such a system requires thorough and 
precise knowledge of all of the enemy and friendly 
aircraft expected to be encountered. A thorough 
analysis of this data may show a general difference in 
the modulation characteristics of friendly and enemy 
aircraft. If this is true, the difference in the modula- 
tion can be used as a basis for identification. 


19.2 SYSTEM AN/APG-15B 

The AN/APG-15B system is a manually controlled 
radar gunsight mounted on the tail turret of a B-29 
aircraft for defensive fire control against enemy night 
fighters. 35 It is taken as a sample system because it 
demonstrates most of the design considerations dis- 
cussed in Section 19.1. The AN/APG-15B was added 
to the B-29 fire-control system after the fire-control 
system was in large-scale production. For this reason, 
it was designed to require the least amount of instal- 
lation engineering and fire-control system modifica- 
tion. The radar has three distinct functions: (1) to 
act as tail warning when the B-29 is flying under con- 
ditions of restricted visibility, (2) to allow reasonably 
accurate blind fire, point-blank, under conditions of 
restricted visibility, and (3) to supply, automatically, 
accurate radar range to the fire-control computer 


d RESTRICTED 


221 


MANUALLY DIRECTED RADAR GUNSIGHTS 


under conditions suitable for optical tracking. Al- 
though it would have been desirable, this system was 
not designed to permit computed blind fire by the 
turret on which the antenna is mounted. This would 
have introduced considerable additional complexity 
into the system, and it was felt that point-blank fire 
w^as adequate for most attacks. However, a simple 
adjustment of the B-29 system makes it possible to 
give a computed lead to a different turret from that 
on which the antenna is mounted. Such an arrange- 
ment was being tested at the conclusion of World 
War II. In this, the lower aft turret delivered com- 
puted fire while the tail turret delivered point-blank 
fire. 

In conjunction with the AN/APX-15 identifica- 
tion equipment, the system is able to identify the 
target as a B-29 or other type of airplane. In the 
operations in which such equipment found general 
use, long-range bombardment of the Japanese home- 
land, this identification was sufficient. The only 


friendly airplanes over the Japanese empire under 
conditions of restricted visibility were B-29’s. 

19.2.1 Arrangement of the System 

Figure 2 is a block diagram of the AN/APG-15B 
and AN/APX-15 equipment. The antenna assembly 
is mounted directly on the tail turret and harmonized 
with the guns so that the radar axis remains parallel 
with the boresight for all turret positions. 32 It is con- 
nected to the receiver-transmitter by a flexible r-f 
cable and to the junction box for power and phase 
reference voltages. The receiver-transmitter is an 
individually cased, self-contained unit which con- 
tains the complete transmitter and receiver, and 
power supply. The transmitter operates on approxi- 
mately 2,700 me and generates approximately 1.5 kw 
of radio frequency in a 0.7 //sec pulse with a recur- 
rence frequency of approximately 1,400 per sec. The 
range unit is an individually cased unit containing 
all the circuits necessary for detecting a target echo 


TO COMPUTER 
AND 

PEDESTAL SIGHT 


<f^u/n//iniiiniminn 


OPTICAL RANGE | 
OPTICAL RANGE REF | 
RANGE | 
RANGE REFERENCE 4 


IDENTIFICATION 

UNIT 


IDENTIFICATION 

LIGHTS 


at 


/ 

/ 

/ 

LLUUi 


mum n // / i in 11/11/ / / /> 1 1 / i m i m / // 

s 


RECEIVER 

TRANSMITTER 


ANTENNA 

ASSEMBLY 


RF 

]zzzzzzzzzzzz£ 

m 


PHASE REFERENCE 


RANGE UNIT 




COMM- 

UTATOR 


MOTOR 




/ 


zzzzzzzzzz 


zzzzzzzzzz 


^TRIGGER 
^ VIDEO 
f A VC 


i RADAR RANGE 
♦RANGE REFERENCE 
t CLAMP 

♦sweep CONTROL 
^DEMODULATED VIDEO 



MOUNTED ON GUNS 


H 


' 1 . 111 11 1 \ 


ZZZZZZZZZZ 

/ 


IDENTIFICATION 


JUNCTION 

zzzzd B0X 




AUDIO 


TO AIRCRAFT POWER 


3aZZZZZZZ2ZZZ2ZZ2^> 


CONTROL 

BOX 




11/11 l 111 11 1 1 1 1 111 1 1 1 1 


SWEEP CONTROL 
RANGE RELAY 
.POWER 


TT 

/ 

/ 

/ 

11 / 1 1 11 1 LJ ^ AUDIO 


/ 


t RADAR RANGE 


INDICATOR 

AMPLIFIER 


INDICATOR 


/ i PHASE REFERENCE 
/^DEMODULATED VIDEO 
-fzJjtCLAMP 

3^7 ^DEFLECTION VOLTAGES 


]/ IZZZZZZZZZZZZZZZZZZZZ i i l 11 1 / ii irm -rn 1 1 n / n / 7y| t CLAMP 

Figure 2. Block diagram of AN/APG-15B and AN/APX-15 systems 



SYSTEM AN/APG-15B 


225 


and measuring the range of the echo. It receives a 
synchronizing pulse and the output from the receiver 
from the transmitter-receiver unit, and in turn, sup- 
plies automatic volume control [A VC] voltage to the 
transmitter-receiver unit. The outputs of the range 
unit are: a voltage proportional to the range of the 
target, reference voltages for measuring range, a 
voltage which varies when a target is found, and the 
modulation from the signals being range-tracked. 
The indicator-amplifier is a separately cased unit 
whose principal function is to compare the phase of 
modulation on the signal being tracked with the me- 
chanical phase of rotation of the antenna assembly, 
to determine the direction and amount of the point- 
ing error. This information, together with the radar 
range of the target, is assimilated in the indicator- 
amplifier and converted into suitable voltages for the 
indicator. The indicator-amplifier also supplies an 
amplified version of the target echo modulation to the 
identification unit, where it is examined for frequen- 
cies which can arise only from B-29 propellers. The 
indicator is small enough to be clamped directly to 
the pedestal sight and the precise angle information 
and rough range information is presented on a 2-in. 
cathode-ray tube. The presentation is type G. 

Besides being the power and information distribu- 
tion center, the junction box contains relays whose 
operation determines the type of range information 
fed to the computer. Under normal conditions, the 
computer receives optical range while the radar set 
is not locked on a target. When the range unit es- 
tablishes that a target has been found, the relays 
automatically switch the computer range input from 
optical to radar. Identification of the target being 
tracked is indicated by a light mounted in view of the 
tail gunner. The identification light is turned on auto- 
matically if the target being tracked is a B-29; the 
light does not turn on if the target being tracked is 
not a B-29. 


19.2.2 Antenna Characteristics 

The antenna assembly is designed to give reason- 
ably large coverage as well as fairly high angular 
accuracy. The antenna feed itself is a standard cur- 
rent-fed dipole and disk; the reflector is a 13-in. parab- 
oloid offset 43^ degrees from the axis of the antenna 
feed and rotated about the antenna feed. The result- 
ing pattern gives an azimuth coverage of approxi- 
mately 30 degrees and an elevation coverage of ap- 
proximately 35 degrees. The one-way crossover is at 
approximately 70 per cent power, resulting in a 
power sensitivity of approximately db per degree. 
The antenna assembly paraboloid is rotated at a gov- 
erned speed of 2,150 rpm. This speed is selected so 
that the fundamental modulation frequency and the 
second harmonic of the modulation frequency will 
exactly straddle the band of modulation frequencies 
expected from a B-29 propeller. In this way the iden- 
tification unit can identify a B-29 simply by passing 
the frequency band 2,400 to 3,780 rpm. 


19.2.3 Operational Performance 

Only one combat wing of B-29 aircraft was 
equipped with AN/APG-15B. However, the experi- 
ence of this wing, together with test results obtained 
in this country, 28 permits a reasonable estimate of the 
performance of the equipment corresponding to the 
three basic functions of the system outlined above. 
The performance is as follows: reliable detection 
range on fighter, 1,500 yd; angular accuracy within 
1,000 yd, approximately }/% degree; range accuracy 
between 1,300 and 300 yd (the range interval accept- 
able to the computer), 25 yd. During the first three 
months of combat the equipment was operational on 
the average 65 to 70 per cent of the time. An analysis 
of the field report indicates that this low figure came 
from other causes than engineering design. 



Chapter 20 

COMBINED OPTICAL AND RADAR FIRE CONTROL 


20.1 INTRODUCTION 

20.1.1 Advantages of Range-only 

Systems 

In this chapter those fire-control radar systems are 
discussed which are primarily meant to provide range 
only; the associated gunlaying systems depend upon 
optical tracking for directional data. This division of 
responsibility between radar and the human eye has 
certain advantages, for although the eye is very in- 
sensitive to changes in range, it is highly sensitive to 
changes in angular position. Radar, on the other 
hand, measures range accurately with a simple com- 
pact system, whereas it requires considerable bulk, 
especially in the antenna structure, to achieve an 
angular accuracy even approaching that of the hu- 
man eye. Thus in the radar-optical systems each 
component is used to its best advantage. 

It is true that ranging to ground targets does re- 
quire that the radar have directional properties, in 
order to pick out the desired target signal from a 
group. The requirements imposed on the directional 
device are much less severe for this application than 
for complete gunlaying systems, and greatly simpli- 
fied techniques have been developed. Such tech- 
niques may be adapted to other uses, notably to 
night-fighter interception and attack (see Section 
20.7.3). 

20.1.2 The Application of Range- 

only Systems 

The tactical applications of range-only systems 
can be divided into the following classes. 

Air-to-Air 

This application includes systems for use by fighter 
aircraft in attacks on other fighters or bombers, as 
well as by bombers trying to shoot down attacking 
fighters. The need for range data is twofold: to in- 
form the gunner when the target has approached 
near enough for him to start effective fire, and to feed 
range data into a computing gunsight which will per- 
mit introduction of ballistic corrections and deflection 
shooting. 

Radars developed to provide range information 
for air-to-air firing are called airborne range only 


[ARO] systems. AN/APG-5 is the example to be 
discussed in this chapter. 

Air-to-Sea 

This class includes attacks from fighters or fighter- 
bombers on ship targets. (A similar problem is pre- 
sented by isolated targets on the ground.) The air- 
plane may use cannon, rockets, machine gun fire, or 
bombs. The radar again is needed to determine 
whether or not the target is sufficiently close to make 
an effective attack possible, and to supply range data 
to a computing device. The chief function of the 
latter is to provide the ballistic corrections needed to 
offset the projectile drop in long-distance attacks. 
The range data are usually fed continuously into the 
pilot’s optical sight. 

In bombing from low altitudes, the range informa- 
tion supplemented by data on altitude and ground 
speed permits determination of the release point 
with the aid of a very simple computer. 

The AN/APG-13A or Falcon system is used pri- 
marily against isolated water targets which give 
strong echoes. It requires an operator to follow the 
target range presented on a scope. The systems men- 
tioned for use against ground targets can also be 
used under these conditions, but are not limited to 
them. 

Air-to-Ground 

This involves attacks on ground targets by fighters 
or fighter-bombers, having the same armament as in 
the previous case. (Ranging on one out of a multitude 
of ships presents a similar problem.) Here the Falcon 
set, originally developed for attacks on ships, be- 
comes inoperative because of the multiplicity of 
targets. However, simple systems using a conical 
scan have been developed which provide range on a 
selected target. 

Such systems are AN/APG-13B or Vulture, which, 
like Falcon, requires an operator and AN/APG-21, 
Terry, which is automatic. 

20.2 THE LIGHTHOUSE TRANS- 
MITTER-RECEIVER [LHTR] 

20.2.1 Choice of R-F Unit 

The lighthouse transmitter-receiver [LHTR] is the 
basic r-f unit now used in most combined optical- 


RESTRIGTED 


226 


THE LIGHTHOUSE TRANSMITTER-RECEIVER [LHTR] 


227 


radar fire-control systems. 23> 24 ’ 43 ’ 44> 131 135 The unit is 
a compact and light (18 in. long, 10 in. in diameter, 
28 lb) r-f head, made possible by the development of 
the lighthouse tube. It operates at S band and is pres- 
surized for high-altitude work. As large antennas 
cannot be installed very well in aircraft of the type 
that would use this equipment, the choice of S band 
resulted in fairly wide beams, of the order of 30 de- 
grees. In the original applications this did not con- 
stitute any disadvantage, because the systems were 
meant simply to determine range of isolated targets 
and because acceptable maximum ranges could be 
obtained with the LHTR unit. 


having four connections, namely, power input, trig- 
ger output for indicators, r-f output for connection 
to the antenna, and video output for connection to 
the indicator. With the exception of the r-f line all 
connections enter through the same cable. 

The unit contains a self-excited modulator, an r-f 
transmitter, a transmit-receive [TR] switch, a local 
oscillator and associated plumbing for these, a crystal 
detector, a receiver strip which includes an i-f ampli- 
fier, a second detector and video amplifier, and the 
necessary power supplies for these parts. 

The lighthouse tube is basically a normal triode; but such 
novel ideas have been used in its construction and in the asso- 


20.2.2 Description of the 

LHTR Unit 

The LHTR unit is a completely self-contained 
transmitter-receiver. It may be described as a box 


ciated plumbing that operation down to S band became possi- 
ble. The tube is of the “flat-electrode” type, using plane sur- 
faces for cathode, grid, and anode. Because the spacings have 
been reduced to very small values, with a corresponding re- 
duction in the transit time, it will operate with good efficiency 
in this frequency band. 



FROM 



A B 

Figure 1 . A. Lighthouse tube and r-f cavity. In this figure, © is the grid coaxial line, ® is the coupling region from the 
feedback coaxial line © to ®; ® is the coupling region from plate coaxial line © to ® ; ® and ® are also part of the plate coax- 
ial line; note that ® changes its axial dimension as the plate rod is moved in and out for tuning. B. LHTR r-f deck assembly. 

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228 


COMBINED OPTICAL AND RADAR FIRE CONTROL 


The tube and cavity are shown in Figure l. 136 The grid is a 
mesh stretched across the opening of a flat circular disk to 
which the glass envelope is sealed on both sides. Anode and 
cathode are part of, and supported by, cylindrical pieces which 
act as center conductors for two sections of concentric trans- 
mission line shown in the same figure. The outer conductor for 
these lines is formed by a cylinder which is slipped over the 
grid disk and supported by it. The cylinder in turn acts as the 
inner conductor of a transmission line for which the outside 
wall of the cavity acts as the outer conductor. This latter line 
feeds back energy from the anode line to the cathode line, and 
thus provides regeneration. Grid bias is supplied through fin- 
gers which make contact with the grid cylinder. Energy is 
taken from the cavity by means 'of a probe; its position de- 
termines the degree of coupling. The general layout for trans- 
mitter and receiver cavities is the same. The transmitter cavity 
is directly coupled to the r-f output line which in turn connects 
to the antenna. The local oscillator is coupled into the crystal 
which is connected to the r-f output via a TR located in a sepa- 
rate cavity. Tuning of the transmitter or local oscillator is 
accomplished by moving the large diameter plate rod which 
slips over the plate cap. Both tuning controls are brought out 
through the pressurized can; the local oscillator tuning can be 
operated by hand during flight. The r-f layout is shown in 
Figure IB. 



Figure 2. Block diagram of LHTR. 

The modulator is a conventional self-excited blocking oscil- 
lator using a 0.75 *isec delay line. Pulse transformers are used 
in the blocking oscillator and in the output coupling. The 
pulse recurrence frequency is approximately 1,200 c. 

The received signals are converted in the crystal detector 
and amplified in the i-f amplifier. The latter is of the triple 
stagger-tuned type and has a bandwidth of approximately 
8 me. Provision is made for manual or automatic gain control. 

A block diagram shown in Figure 2 further illustrates these 
points. The complete LHTR unit is shown in Figure 3. The 
total weight of the equipment is approximately 28 lb; its 
power consumption is 36 watts at 28 v direct current and about 


200 watts at 110 v, 400 c alternating current. Its peak power 
output is about 1.5 kw. The unit has proven itself reliable in 
operation and has a good degree of frequency stability; in fact 
manual adjustment of the tuning can, as a rule, be omitted 
during operation. 





Figure 3A. Top view of LHTR. 

Figure 3B. Side view of LHTR. 

Figure 3C. Bottom view of LHTR. 

Figure 3D. Enlarged view of deck of LHTR. 



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ARO (AN/APG-5) 


229 


20.3 ARO (AN/APG-5) 

20.3.1 Function of the AN/APG -5 
System 

Airborne range only [ARO] systems 35> 37> 132> 136 were 
developed to furnish range data to computing sights, 
as well as to tell the gunner when the target is within 
firing range. They were intended for gun turrets in 
bombers, but as the war progressed, more and more 
attention was given to their use in fighter planes. 

The AN/APG-5 system was planned for firing at 
other airplanes at high altitudes. Under these con- 
ditions the number of radar targets is very small and 
it is sufficient to use a fairly nondirective system in 
which separation between the targets is made on the 
basis of range only. In fact, the lack of sharp radar 
definition is of some advantage, because it permits 
the system to pick up targets and lock on them even 
if the antenna is not pointing exactly at the target. 
This means that the antenna can be pointed along 
the gun line and need not be offset by the lead angle, 
as would be required for an antenna with a narrow 
beam. The large signal obtained from ground return 
might cause the system to lock on the ground; for 


safe operation, the system should be used at altitudes 
which exceed the maximum range of the set. Con- 
versely, this property opens some possibilities of us- 
ing the set as an absolute altimeter over flat terrain. 

Fundamentally, the system uses a set of two con- 
secutive gates which are being swept through the 
complete range under control of a search circuit. 
When these gates intercept a signal, they disconnect 
the searching circuits and lock the timing of the two 
gates; consequently, the signal will fall partly in the 
early gate and partly in the late gate. The gate will 
then stay locked on the target as long as it stays 
within the range of operation of the set (approxi- 
mately 200-2,000 yd). The operator can move the 
tracking away from any undesired target by operat- 
ing a switch which causes the gates to lock on the 
next nearer or farther target. 

20.3.2 AN/APG -5 System Arrange- 
ment and Operation 

The system consists of the following parts: an- 
tenna, LHTR, range unit, control box, in-out switch, 
on-target light, junction box, power supply, and 
cables. Figure 4 shows these parts. The range follow- 


RECEIVER-TRANSMITTER 

RT-39/APG-5 


SERVO-AMPLIFIER UNIT 
AM-25/APG-5 


CONTROL BOX 
C-86/APG-5 


TARGET SELECTOR 
SWITCH 



TARGET 

INDICATOR LAMP 


PROTECTIVE CAP 


ANTENNA 

AS-68/APG-5 


Figure 4. Components of ARO system. 


230 


COMBINED OPTICAL AND RADAR FIRE CONTROL 


up unit and gear box are shown, although they are 
not a part of the radar system. The total weight is 
95 lb including power supply. The power consump- 
tion is 450 watts at 110 v, 400 c alternating current 
and 85 watts at 28 v direct current. A block diagram 
is shown in Figure 5. 



Figure 5. Block diagram of ARO system. 


The antenna is an 18-element end-fire array. It 
consists of a section of coaxial line with a set of 
dipoles for each element of the array. These are ar- 
ranged to intensify the field in the forward direction 
and cancel it in the reverse direction. The beamwidth 
is approximately 30 degrees. The whole unit is en- 
closed in a cylindrical, pressurized plastic housing. 

The range unit contains the circuits which provide 
search, tracking, switchover from one to the other, 
and automatic volume control [A VC] for the LHTR. 
The action of the range circuit can best be under- 
stood from the block diagram, shown in Figure 6. 
The LHTR modulator triggers a delay multivi- 
brator. The trailing edge of the pulse generated in 
this multivibrator can be moved in and out, under 
the influence of the grid bias on one of its tubes. 
Apart from the first few microseconds, the delay be- 
tween the trigger and this trailing edge is a linear 
function of the bias voltage. After amplification and 
differentiation this trailing edge controls the start of 
the very narrow (0.7 /usee) early gate. Its trailing 


edge, in turn, controls the start of the equally narrow 
late gate. These gates are applied to the coincidence 
tubes. The video signals, after passing through a de- 
lay line which absorbs the few microseconds over 
which the delay multivibrator is nonlinear, are also 
applied to these coincidence tubes. The coincidence 
tubes can only conduct when the gate and the video 
are applied simultaneously. 

In the search condition, the bias voltage for the 
delay multivibrator is derived from a saw-tooth gen- 
erator which sweeps the gates through the total 
range of the system. Whenever coincidence is ob- 
served between a signal and the gates, a signal is 
transmitted to the detector circuit and amplified in 
the d-c amplifier. This operates the disconnect relay 
and starts the unit tracking. As the search sawtooth 
has been applied through the range condenser in the 
integrator unit, the initial range voltage on this con- 
denser is already established, and the only problem 
left is to keep this voltage locked on the target. This 
is done by the coincidence tubes. If the voltage on the 
range condenser is larger than that corresponding to 
the actual target range, most of the video signal will 
fall in the early gate. This will operate the discharge 
tube in the integrator, thereby decreasing the range 
voltage; the reverse holds if the voltage on the range 
condenser is smaller than that corresponding to the 
correct range. 


_r*te -Lsf- 



Figure 6. Block diagram of ARO range unit. 

When the sight is of a type that cannot accept this 
range voltage, the latter is delivered to the follow-up 
unit. This unit contains a differential amplifier sys- 
tem which controls two thyratrons. These provide 
power to the motor located in the gear box and con- 




FALCON (AN/APG-13A) 


231 


trol it in such a way that it provides a shaft rotation 
proportional to range, which operates the lead-com- 
puting sight. 

The control box contains the power switches, fuses, 
and pilot lights for the 110-v and 28- v lines, as well 
as a switch which permits manual ranging. The on- 
target light and the in-out switch are each provided 
with a long cable to permit installation in the most 
convenient spot for either gunner or pilot. 

20.4 FALCON (AN/APG-13A) 

20.4.1 Function of the Falcon 

System 

Falcon 33, 34> 40_42 ’ 120 > 122 - 123 - 126 * 131 was originally 
developed for use with cannon-equipped B-25 air- 
planes. At an early stage of this work it was found 
that the destructive range of the cannon (approxi- 
mately 6,000 yd) far exceeded the range (about 
1,500 yd), over which hits could be obtained by 
point-blank firing. This is caused by the ballistic drop 
which the projectile suffers in its trajectory. Under 
these conditions the advantage of the heavy cannon 
did not appear to justify carrying the extra weight. 
Yet it seemed fundamentally sound to carry a piece 
of long-range armament to surprise the enemy, kill- 
ing his antiaircraft crews or forcing them to take 
cover, then strafing or bombing in comparative 
safety. Since a flexible gun mount is impractical in an 
airplane, it was decided to give the gun a fixed 
mounting, and to achieve the required superelevation 
by superelevating the whole airplane by the desired 
amount. This can be done if the pilot’s optical sight 
is depressed by the angle corresponding to the bal- 
listic deflection. Since airspeed, initial range, and 
altitude can be held fairly well to prescribed values 
by the pilot, the range to the target is the parameter 
which has the major influence on the ballistic cor- 
rection in this problem. As explained before, the hu- 
man eye is a poor judge of range, so that radar seems 
the natural solution. 

The set which provides the range data for this 
problem is Falcon or AN/APG-13A. It uses the 
LHTR which feeds its video signals into an A scope 
equipped with a precision sweep. A precision range 
marking “step” is provided, which the operator can 
adjust so that it matches the target echo. In this op- 
eration of matching the echo, the operator cranks the 
desired range data into the gunsight (NC-2) where a 
ballistic cam translates the data into the proper 


sight-depressions. Little objection was raised to the 
requirement for an operator in the case of the B-25, 
which normally carries four or five men. The results 
with this system have been highly satisfactory, as 
shown by an Army report. 122 It was possible to obtain 
60 per cent hits on a target 100 ft wide and 50 ft high 
from ranges between 4,500 and 3,500 yd, and 67 per 
cent hits for ranges between 3,500 and 2,500 yd. 

During the operational use of this system it was 
found that deviations from the airspeed for which the 
sight cam was designed could not be neglected. More- 
over the assumption of constant airspeed during the 
run does not conform to actual conditions, as the 
pilot may have difficulty adjusting his throttle during 
the approach dive. Some corrections can be made by 
an intelligent use of the slope and zero adjustments 
which are provided in the indicator sweep. A study of 
this was made in connection with the Vulture system 
and is discussed under “Ballistic Cams” in Section 
20.5.1. 

20 . 4.2 Falcon System Arrangement 
and Operation 

The general arrangement of the system in the air- 
plane is shown in Figure 7, the components in Fig- 
ure 8, and the block diagram in Figure 9. The basic 
r-f unit is again the LHTR. Its r-f output is connected 
to an end-fire array antenna which transmits and re- 
ceives the signals. This type of antenna has the ad- 
vantage of low wind resistance and easy mounting 
and provides sufficient directivity for the purpose 
(about 30 degrees). 

At the transmission of each r-f pulse, a trigger 
pulse is sent into the A scope where it triggers a 
linear sweep. The operator observes the received sig- 
nal and matches it with a range marker. This range 
marker has the form of a step in the base line, which 
the operator brings into coincidence with the signal 
by turning a hand crank. This matching can be done 
with great accuracy. 

The step voltage is obtained in the following way. 
A potentiometer is connected to a shaft in the optical 
sight head which also carries the ballistic cam. This 
shaft can be turned by means of the hand crank 
shown in the block diagram and is arranged to have 
a rotation proportional to range; thus, the voltage on 
the potentiometer arm will be proportional to range. 
This voltage is compared with the sweep voltage, and 
at the time when the sweep voltage reaches equality 
with it, a step in the base line is initiated. Thus, the 


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232 


COMBINED OPTICAL AND RADAR FIRE CONTROL 



Figure 7. General arrangement of Falcon system in an airplane. The radar components and men are shown dis- 
proportionately large. 



Figure 8. Components of Falcon system. 


v : ,. . 

RESTRICTED 


FALCON (AN/APG-13A) 


233 



Figure 9. Block diagram of Falcon system. 


operator can crank in range by rotating the camshaft 
until the step and the signal match. By so doing he 
has automatically cranked the proper ballistic de- 
pression into the pilot’s sight. As this is a continuous 
process, the pilot can fire at any time during the run 
after the radar operator has told him that he is track- 
ing. 

Obviously a high degree of linearity is needed in 
the sweep. The primary parameter used in measuring 
radar range is time, but if the sweep is truly linear, 
measurement of a voltage may be substituted for the 
measurement of time. This is done in the Falcon sys- 
tem by comparing the voltage on the sight potenti- 
ometer with that of the sweep. Therefore, if the sweep 
is nonlinear, erroneous range measurements will re- 
sult. Considerable work was done on this problem by 
the indicator group at the Radiation Laboratory and 
a very nearly linear sweep circuit resulted. Descrip- 
tions of these circuits will be found in the Radiation 
Laboratory Technical Series books on indicators. It 
will also be clear that the linearity of the sight po- 
tentiometer must be as good as that of the sweep, but 
this is a fairly simple manufacturing problem. 

The indicator, a modified A scope with range step, 
is called an M scope. Apart from the sweep and step 


circuits, it has the usual video amplifier circuits, pro- 
vision to mix the video and the step voltages, and to 
blank the cathode-ray tube beam during the unused 
time intervals between r-f pulses. A built-in set of 
range marker pips is provided for calibrating the op- 
eration of the range measuring circuits during flight. 
An additional long-range sweep (24,000 yd) is pro- 
vided. 

The pilot’s sight has an optical system which pro- 
duces a collimated image of a target bead and sur- 
rounding ring on a piece of plane-parallel glass, inter- 
posed at an angle in the pilot’s line of sight. Variation 
of the angle of this glass plate will change the eleva- 
tion of the target bead. The glass is mounted on a 
shaft which is spring-loaded and has an arm resting 
with a point on the ballistic cam. Thus, rotation of 
the camshaft will introduce the proper ballistic cor- 
rection. The relative orientation of the cannon with 
respect to the reference line of the airplane must be 
determined with great care, and the optical sight 
must be carefully harmonized with the cannon. 
Proper harmonizing charts and procedures for this 
purpose have been developed. 

With the Falcon system the following operations 
take place during an actual attack. Having checked 


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234 


COMBINED OPTICAL AND RADAR FIRE CONTROL 


the correct operation of the set, the radar operator 
will make sure that the range dial has been cranked 
to the maximum range. At fairly long ranges (7,000 
or 8,000 yd) the pilot will begin to fly his airplane so 
that the bead of his sight is centered on the selected 
target, and will inform the radar operator of this fact. 
As soon as the radar operator has identified the target 
signal on his scope, he will either wait until this signal 
moves in (to the left) so as to coincide with the range 
step at maximum range, at which time he will begin 
cranking; or if the target signal has already moved 
below the maximum range, he will crank the range 
step to the left until it has caught up with the target 
echo. In either case he will continue to crank at such 
a rate that the range step moves to the left in coin- 
cidence with the target signal, and will inform the 
pilot that firing can commence. From there on, the 
pilot can fire as often as the cannoneer can load. 

As the closing speed of the airplane is reasonably 
constant in such an approach, it seemed possible to 
relieve the radar operator of some of the burden of 
manual cranking. For this purpose an aided-tracking 
box was developed. It contains a constant-speed 
motor and a variable-speed (ball and disk) transmis- 
sion. The operator sets a knob at the indicated air- 
speed and releases the mechanism when coincidence 
of step and echo is obtained. Provision is made to feed 
in the corrections needed to compensate for accelera- 
tion during the approach. 

20.5 VULTURE (AN/APG-13B) 

20.5.1 Function of the Vulture 
System 

By the time that the Falcon system began to be 
operationally available, the tactical situation had 
changed so much that its primary targets, isolated 
ships, were hardly to be found within the operating 
ranges of the cannon-equipped airplanes. At the same 
time the use of cannon fire on land targets became 
increasingly important and it was desired to employ 
the B-25’s equipped with Falcon for this purpose. 

Unfortunately, when Falcon is operated over land, 
many echoes appear on the indicator and the radar 
operator cannot easily determine which echo repre- 
sents the target at which the pilot is pointing his 
plane. An improved type of radar is therefore needed. 
It must combine a fairly good directional sensitivity 
with a method of indicating the particular echo 
toward which the airplane is being aimed. 


Vulture or AN/APG-13B 25 - 38 - 39 - 50 was designed 
to meet these requirements. 

The use of a conical-scan gunlaying radar system 
(see Section 18.1.2 and Figure 2, Chapter 18) had 
been previously proposed as a solution to the air-to- 
ground ranging problem. Such a system, however, 
requires a range gate, set at the correct range, before 
an on-target indication can be obtained and thus re- 
quires knowing the answer before attacking the prob- 
lem. In principle there are two solutions. One is to 
make a range gate that continuously searches in 
range and only locks the system when both range and 
directional information give an on-target indication. 
This is the basis for the system known as Terry 
(AN/APG-21). The other method is to present a 
simultaneous panoramic view of all target signals 
within the radar beam, together with a clear indica- 
tion of whether or not such targets are on the scan- 
ning axis of the rotating lobe system. This will indicate 
which of the many targets is the one at which both 
pilot and radar antenna are looking. (A linkage must 
be provided to keep the pilot’s sight axis and the 
scanning axis parallel even though the former is de- 
pressed to provide the ballistic corrections.) This 
second approach is used in the Vulture system; its 
basis is the V scope presentation described below in 
Section 20.5.2. The military operation of this system 
is fundamentally the same as that of Falcon; the 
pilot aims the airplane at the desired target, the 
radar operator recognizes this target on the scope 
and tracks it with a range marker. This tracking op- 
eration causes a ballistic cam to depress the pilot’s 
sight by the amount needed for the ballistic correc- 
tion and another cam to depress the scanner axis by 
the same amount. 

Aided-Tracking Box 

In range-tracking with Vulture the operator loses 
the signal more easily than with Falcon, particularly 
when the pilot is unable to hold the target in the 
center of his sight. This will occur when the airplane 
is not flying smoothly in a straight course. 

To overcome this difficulty and to provide 
smoother tracking than can be obtained with the 
hand crank, an aided-tracking system was developed. 
The planned airspeed is set in beforehand; when the 
operator pushes a button, the range marker moves 
across the scope at the preset speed. A slewing switch 
is provided, which enables the operator to bring the 
range marker quickly into coincidence with the target 
signal at the beginning of the run. If the operator 




VULTURE (AN/APG-13B) 


235 


makes no further adjustment, the range fed to the 
sight cam may gradually deviate from true range; 
but since the ballistic corrections approach zero as 
range decreases, good results can be obtained in this 
way. However, the operator has a knob at his dis- 
posal by which he can make minor adjustments to 
take care of changes in closing speed during the run. 
The pilot and the scanner must continue to look at 
the same target as the sight glass is tipped. Normally 
this would require a servomechanism to make the 
scanner axis of rotation follow the depression of the 
pilot’s sight. To obviate the weight and complexity 
which this would introduce, the tracking box was 
given sufficient power to drive both the sight and the 
scanner through flexible cables. 

Other System Improvements 

Receiver overloading will spoil the type V indica- 
tion and must therefore be avoided. A time sensitiv- 
ity control (gain expansion) is installed in the re- 
ceiver and eliminates the necessity for continuous 
checking of the gain setting otherwise required to 
prevent overload. 

One of the major problems involved in the use of 
these systems is that of boresighting the scanner — 
that is, assuring that the axis of rotation of the scan- 
ner is parallel to the line of sight. This was simplified 
by arranging the scanner mount to permit substitu- 
tion of new scanners, which are preboresighted elec- 
trically. Reharmonizing is then the only operation 
required in the field. 

Ballistic Cams 

Extensive work was done on the ballistic cams. 
The cams in the Falcon system were developed for 
250 mph airspeed, which proved to be excessive for 
the B-25 aircraft. Constant speed during the ap- 
proach was assumed, which would require juggling 
of the throttles during the run. Attempts were made 
in the Falcon system to get better target scores by the 
use of zero and slope corrections, that is, by changing 
these parameters in the indicator sweep to correct for 
the difference between actual conditions and those 
assumed when calculating the cams. New cams were 
finally designed which assume the more reasonable 
cruising speed of 225 mph and constant throttle set- 
ting during the approach, a condition preferred by 
pilots. 

Allowance for several other factors, such as varia- 
tions in initial airspeed, initial range, initial altitude, 
altitude of target above sea level, and temperature 


was made. The Applied Mathematics Panel coop- 
erated in the design of a circular slide rule which 
allows zero and slope correction for conditions differ- 
ent from the standard approach to be set in before 
the run (see Section 21.2.1). Cams using the AMP 
data are used in the Vulture systems for both scanner 
and pilot’s sight. 72 Firing tests showed that the same 
high accuracy could be obtained under nonstandard 
conditions with the slide rule corrections as was previ- 
ously obtained under the standard conditions for 
which the cam had been designed. 

Adaptation for Rocket Fire 

The original plans assumed the use of the 75-mm 
cannon. As more progress was made in rocketry, 
rockets took over a major part of the tactical tasks, 
even though the cannon was still the answer to some 
specialized problems, such as long-range high-pre- 
cision firing (for instance into the entrance of tun- 
nels). Further discussion of the use of Vulture and 
related systems for rockets is given in Section 21.2. 

Performance 

The Vulture system was extensively tested by the 
Army ; 124 unofficial information seems to indicate 
that the results were very satisfactory. Range ac- 
curacy is shown in Figures 10, 11, and 12, which give 
the relation between the range as determined by the 
radar operator and the range determined by photo- 
graphic means. It can be seen that the operator 
noticed deviations and started to put in rate correc- 
tions when the deviations became appreciable. In 
Figure 12, firing tests were being conducted with a 
target altitude of 4,300 ft above sea level; and the 
slide rule corrections of —270 yd at 5,000 yd, and 
— 100 yd at 1,000 yd were being used to compensate 
for the deviation from the standard approach. The 
circles on the graph (“true radar range”) represent 
the actual range-dial readings with the calibration 
corrections subtracted out. 

Twenty-four “crash program” units had been com- 
pleted at the end of the war, complete engineering 
data and a manual were available, and an Army 
directive had been issued to convert all Falcon sys- 
tems into Vulture. 

20.5.2 Vulture System Arrange- 
ment and Operation 

The system consists of the following parts: an- 
tenna assembly, scanner mount, LHTR, indicator, 


RESTRICTED '' 


236 


COMBINED OPTICAL AND RADAR FIRE CONTROL 



RANGE DIAL XXXXX 
GROUND TARGET 

Figure 10. Range accuracy of Vulture system. Radar 
range is shown by crosses, photographic range by un- 
broken line. 

aided-tracking box, pilot’s sight, power supply, elec- 
trical and mechanical connection cables. Figure 13 
shows some of these. The weight is 135 lb including 
power supply; the power consumption is 225 watts 
at 115 v, 400 c and 110 watts at 28 v direct current. 
Further description of these parts is given below; a 
block diagram is given in Figure 14, a sketch of the 
installation in Figure 15. 

Antenna 

The antenna assembly is the same as that in the 
AN/APG-15, but the spherical housing and the 
brackets of the AN/APG-15 are omitted. Instead, 
the antenna assembly has been equipped with two 
trunnion bearings. One of these carries the cam- 
follower bearing and the boresight adapter which is 
lined up with the scanner boresight axis during the 
electrical boresighting. The trunnion arrangement 
makes it easy to change scanners in the field. As the 
electrical boresighting has been done previously, it is 
sufficient to insert an optical boresighting tool in the 
adapter and to reharmonize. The latter is done in 
elevation by adjusting the zero set on the cam fol- 
lower and in azimuth by moving the scanner mount. 

The scanner mount carries the antenna by means 
of the trunnions, which in turn allow for movement 
of the antenna assembly in elevation. A spring pulls 

the antenna assembly so that its cam-follower bear- 

* 



Figure 11. Range accuracy of Vulture system. Test 
conducted by Army, range accuracy analyzed at Radia- 
tion Laboratory. 


ing is resting on a ballistic cam located in the scanner 
mount. This cam is driven by the aided-tracking box 
through a flexible cable which connects to a worm re- 
duction in the scanner mount. The cam is cut in such 
a way that during the approach the spring tension 
will help the aided-tracking box. This reduces the 
load and improves the accuracy. The scanner mount 
is equipped with a junction box for the cable that 
brings d-c power to the scanner and also carries the 
scanner synchronizing impulses. 

In the World War II installations of the Vulture 
system the antenna was housed in a nacelle protrud- 
ing under the fuselage of the ship. This was done for 
reasons of expediency; a nose installation would have 
been preferred if sufficient time to engineer it had 
been available. The length of r-f and mechanical 
drive cables was approximately 15 ft. 

LHTR 

The LHTR is a standard unit. The time sensitivity 
control (or gain expansion) of the receiver i-f strip is 
applied through the gain-control lead and does not 
require modification of the LHTR unit. 

Indicator 

The indicator is a modified Falcon indicator, 
equipped to handle either the V presentation or the 
standard Falcon A scope, at the throw of a switch. 
In the V presentation, range is still displayed along 


RESTRICTED 


VULTURE (AN/APG-13B) 


237 



Figure 12. Range accuracy of Vulture system, from a firing run in which ballistic calibrations of —270 yd at 5,000 yd 
and -100 yd at 1,000 yd were used. Tests conducted by Army, range accuracy analyzed by Radiation Laboratory. 



Figure 13. Some of the components of the Vulture system. (Aided-tracking box at far left.) 

RESTRICTEf )' ' " 1 



238 


COMBINED OPTICAL AND RADAR FIRE CONTROL 



Figure 14. Block diagram of Vulture system. 


the horizontal axis. Two range intervals are pro- 
vided, 0-6,000 and 0-24,000 yd; the latter does not 
have a range marker. The video signals have been 
removed from the vertical plates and applied to the 
cathode of the cathode-ray tube, resulting in inten- 
sity modulation as in a B scope. In the vertical direc- 
tion the number of degrees of rotation of the scanner 
is displayed. For the V presentation it was found 
desirable to display two full revolutions of the scan- 
ner (720 degrees), hence the vertical saw-tooth gen- 
erator operates at half the revolution frequency of 
the spinner. 

For the one target that is on the axis of rotation of 
the scanner, the average returned signal is independ- 


ent of the position of the scanner and thus stays con- 
stant as the scanner is rotated. For each successive 
pulse, the beam describes a horizontal trace slightly 
above the previous ones, and, thus, a target on the 
scanner axis will produce a series of dots of equal 
intensity, one above the other, each at the correct 
range of the target. On the scope this shows up as a 
vertical line of constant intensity (unmodulated). 

For any targets that are off the axis, the average 
signal intensity will vary as the scanner goes around 
and thus deposit bright spots on some of the hori- 
zontal traces and very weak or no spots on others. 
On the indicator this shows up as a broken (modu- 
lated) line or, in extreme cases, as just two dots. This 


REgTgICTEQ 


VULTURE (AN/APG-13B) 


239 


INSTALLATION OF A PG - I3B IN B~25 AIRPLANE 


1. SCANNER ASSEMBLY 

2. LHTR 

3 M SCOPE 

4 TRACKING BOX 

5 JUNCTION BOX 
6.SIGHT 

7 NACELLE CONSTRUCTION 


OV 400 < v+ 28 V OC 
28V DC 



-MECHANICAL CONNECTIONS 
-ELECTRICAL CONNECTIONS 


Figure 15. General arrangement of Vulture system in an airplane, 
proportionately large. 


The radar components and men are shown dis- 


makes it easy to see which targets are on the axis 
(that is, which one the pilot is looking at), and which 
ones are off. The position along the vertical axis at 
which the target return appears is a direct indication 
of the position of the target in relation to the air- 
plane. See also Section 20.7.3. 

Figure 16 shows some target areas as seen from the 
nose of the airplane during a typical approach, and 
the corresponding indications on the Y presentation. 
It can be seen that a large number of confusing un- 
desired targets are present, and these make it gener- 
ally impossible to recognize the desired target on the 
A scope. Yet there is no trouble in recognizing the 
target in the V presentation. These pictures were 
taken in the early stages of development and neither 
range marker nor gain expansion had been incor- 
porated in the set. The solid line appearing at the 
left of the scope presentation is caused by the initial 
pulse, which thoroughly overloads the receiver. 

From these photographs the reason for the choice 
of two revolutions per scan can also be appreciated. 
If an off-axis signal is received from such a direction 
that only the top and bottom part of the presentation 
show fade-outs, then it would be difficult to judge 
whether the end of the trace was caused by modula- 
tion or because the end of the scanning pattern had 
been reached. If two revolutions are presented, how- 
ever, a clear break will always be visible in some part 
of the trace. 


If two targets are present at exactly the same 
range, one on and one off the scanner axis, then the 
received signal is the superposition of a modulated 
and an unmodulated signal, and no clear unmodu- 
lated trace appears on the scope. Very often, how- 
ever, the desired target can be found by looking for 
the line that has minimum modulation, because any 
such superposition will not show zero signal at any 
point of the trace. Such numbers of confusing targets 
might occur at exactly the same range that recog- 
nition would become impossible. Fortunately the 
tests showed that the number of cases in which the 
system becomes inoperative is small. These condi- 
tions, however, indicate that a sharp beam is a defi- 
nite advantage for the Vulture system, both for the 
reason stated above and because the greater concen- 
tration of energy will permit ranging on smaller 
targets. This is discussed in Section 20.7. 

The indicator is a modified Falcon indicator, shown 
in Figure 17, in which two tubes have been added. 
One tube is a multivibrator which provides the 2 to 1 
stepdown from the spinner frequency and simultane- 
ously generates a saw-tooth at this frequency. One 
half of the other tube acts as a saw-tooth amplifier 
and the other half provides the necessary intensifying 
and blanking signals. 

The following of the target range is again done by a 
step in the base line. As the CRT is now operated as 
a B scope it would be impossible to see any step in the 


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COMBINED OPTICAL AND RADAR FIRE CONTROL 



Figure 16. Air view of typical targets (transformer station, left, and hangar, right) with corresponding presentation 
on the V scope. Photographs were made before the gain expansion (time sensitivity control) and range marker were 
installed. The solid black line at left of scope photographs is caused by the initial pulse. It should be disregarded. 


base line because the beam is normally invisible. To 
overcome this, an intensifying signal is applied at the 
end of each vertical sweep. This intensifies one or two 
lines of the scanning pattern at the bottom of the 
picture. If echoes are received during this period 
they will cause blooming of the picture and thus 
make it impossible to see the range marking step, 
particularly since its amplitude must be kept small. 
For this reason a paralyzing pulse is applied to the 
intermediate frequency, which prevents any signal 
from coming in during this period. Further investi- 
gation showed that erroneous on-target indications 
might be obtained under certain conditions if the 
amplitude of the step was appreciable. Consequently 
the step was limited to low values; an attempt to 
sharpen the step resulted in a small overshoot which 
was found very helpful in operation. 

In this system the eyes of the operator perform a 
number of the functions requiring circuits in com- 
plete gunlaying systems. The human eye is well able 
to compare brightnesses in different parts of the field 
of view, remember positions in the field of view, dis- 
tinguish between stationary and moving impressions, 
and to integrate. This permits the elimination of 
balanced detectors, of comparison circuits and indi- 


cators, and of integration circuits. Because the data 
are presented in proper form, it is possible to obtain 
all the additional functions of the Vulture by the 
addition of only two tubes to the Falcon indicator. 

Aided Tracking 

The range step is generated, just as in the Falcon 
system, by comparing the sweep voltage with the 
range voltage appearing on the range potentiometer 
in the sight head. The operator is assisted by the 
high torque aided-t racking box (described in Section 
20.5.1), which drives both pilot’s sight cam and the 
scanner cam through flexible cables. It is equipped 
with a push-button starter and two automatic stops. 
One will stop the motor when the range has reached 
its maximum (6,000 yd), and thus prepare the system 
for a new run. The other stops it at 300 yd. Its pur- 
pose is to prevent the tracking box from beginning 
the reset operation, which would begin to depress the 
pilot’s sight and thus spoil his aim before he has com- 
pleted his machine gun strafing. 

Some minor modifications make this set usable for blind 
approaches on good radar targets or beacons. They involve 
turning the indicator tube by 90 degrees and changing the 
multivibrator speed-control resistance to obtain a 1-to-l syn- 


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TERRY (AN/APG-21) 


241 


chronizing ratio. Together with the development of the Terry 
system, this makes possible a set which would give search, fire 
control, and AI (Section 20.7). 



Figure 17. Vulture indicator. 

20.6 TERRY (AN/APG-21) 

20.6.1 Function of the Terry 
(Automatic Vulture) System 

This system was developed for single-seater air- 
planes where a separate radar operator cannot be 
carried. Originally this type of airplane was equipped 
with machine guns only ; early radar plans called only 
for ARO systems, used for air-to-air firing over short 
ranges and on isolated targets. 

As the development of rockets progressed and they 
became usable over longer distances (see Sections 

21.2.2 and 21.2.3) it became more and more impor- 
tant to install them on fighter planes and use them 
against ground targets. The use of longer ranges 
brought the same problems of correction of ballistic 
drop as encountered in the airborne cannon, plus 
some new ones peculiar to rockets. In any case, it was 
necessary to have proper range data to feed into com- 
puting devices. Except when steep dive angles are 
used, the determination of the correct range is again 
a serious problem, a solution for which was sought in 
the use of radar. If the pilot is to be free from all dis- 
tracting activities during the actual approach to his 
target, this radar should be entirely automatic. The 
Terry system 26 - 52 meets the requirements for such a 
radar system. Theoretical considerations led to an 


expected accuracy of ± 100 yd at 5,000 yd, although 
this had not been confirmed by actual tests. 

The two fundamental methods of obtaining range 
to a land target from a conical-scan system were dis- 
cussed under the Vulture system (Section 20.5). The 
Terry system uses the method of searching through 
the complete range and of locking on a signal only 
when both the range and angular information give 
an on-target indication. 

As was mentioned, the system searches through the 
full usable target range. When a target is located, the 
range gate tries to lock on the target and then passes 
the problem on to the modulation tracking circuits. 
If the target is the one at which the pilot’s sight and 
the rotation axis of the offset scanner are pointing, 
then the signal will have constant intensity as the 
scanner goes around. In this case the modulation 
tracking circuits will exercise no control at all and 
permit the range gate to stay locked on the target. 
If the target is not the one at which the pilot is look- 
ing, then there will be modulation. The polarity of 
this modulation will indicate whether the target is 
closer in or farther out than the one at which the 
pilot looks. The modulation tracking circuits will 
then develop a voltage which upsets the range gate 
tracking and drives the gates in or out as required. 
This process continues until the correct target has 
been found, after which the range gates will follow it 
in. Complete ARO functions are present in this sys- 
tem and the equipment is accordingly built with a 
switch so that either ARO or Terry operation can 
be used. 

The equipment is particularly adapted to furnish 
range data to a computing sight, such as the Draper- 
Da vis (Army A-l) sight, or the Navy pilot's universal 
sighting system [PUSS] under development at Frank- 
lin Institute. 

The pilot’s sight and the scanner rotation axis 
must again point in the same direction, and, thus, 
the output of the computing mechanism should drive 
the sight and the scanner in synchronism. The pilot 
has only to point his airplane at the target by means 
of his sight and wait until the on-target indicator 
lights up. After that, he can fire at any time during 
the approach. 

At the conclusion of World War II some experi- 
mental models of this system had been built and test- 
flown. Official tests by Army or Navy had not been 
made at that time. A set of pictures taken from the 
nose of the airplane during a typical run is shown to- 
gether with the range dial indications in Figure 18. 


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242 


COMBINED OPTICAL AND RADAR FIRE CONTROL 



Figure 18. Photograph of a target with corresponding range-meter readings taken during a run with Terry. 








TERRY (AN/APG-21) 


243 


Six units of this type had been ordered by the 
Army and the Navy and these units were under con- 
struction at the time the war ended. 

20.6.2 Terry System Arrangement and 
Operation 

The system consists of the following parts : antenna 
assembly, scanner mount, LHTR, ARO range unit 
(modified), modulation-tracking box, pilot’s control 
box, pilot’s sight, power supply, cables, computer 
(may be part of the sight), and servo drive for an- 
tenna. The last two items are not considered to be 
part of the radar system. A block diagram of the unit, 
exclusive of antenna, LHTR, and computer, is 
shown in Figure 19. 

The antenna assembly is the same as that used in 
the Vulture system except that the offset of the dish 
has been increased from 4.5 to 7 degrees, in order to 
increase the angular sensitivity. 

The design of a scanner mount will depend upon 
the particular computer and sight adopted. If the 
lead-angle problem is eliminated and corrections are 
made for ballistic drop only, it is sufficient to drive 
the scanner in elevation. In this case the scanner 
mount of the Vulture system is used. 

If Terry is to be incorporated into a sighting sys- 
tem which does more than merely compensate for 
ballistic drop, then the single ballistic cam is inade- 
quate, and is replaced by a computer of some com- 
plexity. Cams may still be used in the sighting head 


and scanner mount, perhaps with a rise which is a 
linear function of range, to introduce deflection. 
With such a system a servo drive would be required. 
With a lead-computing sight like the A-l, the scanner 
mount must follow the pilot’s sight both in azimuth 
and elevation. An experimental unit of this type was 
developed under an Army contract at the Massachu- 
setts Institute of Technology [MIT]. In this a fixed 
sight and scanner mount were used, as all runs were 
made for photographic recording and none for firing. 

The range unit is modified by having leads brought 
out which make it possible to insert biases in the 
6AC7 early and late gate coincidence tubes and from 
the detector tube located in the range unit. 

The modulation-tracking unit is completely new. 
It detects any modulation that may be present in a 
signal on which the range gates try to lock and de- 
termines its polarity. Referring to Figure 20, let us 
assume that the range gates in the range unit have 
locked on target B, while the pilot’s sight and the 
scanner are pointing at target A. The modulation- 
tracking box will then detect more signal return 
when the scanner is pointing up than when it is 
pointing down. This will set up two bias voltages 
which are applied separately to the 6AC7 coinci- 
dence tubes in the range unit. These biases are of 
such values and polarities that one will cut off the 
late gate tube and the other will hold the early gate 
tube on, so that only the early gate tube acts on the 
range-tracking circuits. This will move the gate 
away from the range of target B and drive it in 


AN/APG-5 RANGE UNIT 



COMMUTATOR 


Figure 19. 


Block diagram of Terry search and tracking units. 



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244 


COMBINED OPTICAL AND RADAR FIRE CONTROL 


UP 



towards target A. The reverse would happen if the 
gates had locked on target C ; they would then have 
been driven out towards A. Thus, the modulation- 
tracking box will drive the gates away, and in the 
proper direction, from any target for which the sig- 
nals in the up and down position of the scanner are 
unequal, and finally leave the gates on the target for 
which these signals are alike. 

The pilot’s control box contains a selector swatch 
which switches the set on and off and selects between 
ARO and Terry operation. Furthermore, it has the 
pushbuttons as used in the ARO as well as a power-on 
pilot light and an on-target light. 

An installation problem peculiar to the Terry sys- 
tem, which would usually be installed in single-seater, 
single-engine airplanes, is that one of the desirable 
locations for the antenna is under the fuselage. Here, 
however, the propeller may intercept part of the 
beam. The degree of modulation which would result 
from such a location and its influence on the tracking 
must still be determined. 

The development of the Terry system together 
with that of the Vulture opened some interesting 
possibilities towards the realization of a universal 
radar set. This is further discussed in the following 
section. 

20.7 FUTURE DEVELOPMENTS 

20.7.1 General 

The cessation of the activities of the Radiation 
Laboratory put an end to further work on a number 
of new ideas in the field of combined optical and 
radar fire control. Although a few of these items were 
of a stop-gap nature, the majority contained ideas 
which definitely merit further investigation and de- 
velopment. Some of the ideas involve further analysis 
of the operation of the system in order to improve its 
performance, some involve combinations of tech- 
niques now known into a new system, and some in- 


volve radically new systems or components. They 
will be discussed in this order. 

20.7.2 Analysis 

An important job of investigation remains to be 
done in the overland systems using conical-scanning 
methods. This is the determination of optimum val- 
ues for the r-f frequency, beamwidth, crossover, and 
time constants. 

The present choice of r-f frequency was dictated by 
the large-scale availability of the S band LHTR unit. 
The experience gained has already indicated that a 
narrower beam is desirable, while the antenna size 
should, if anything, be reduced. Thus the frequency 
should at least be increased to X band; the possible 
desirability of K band should be investigated. 

Too great a beamwidth causes the r-f energy to be 
spread out over too large an area and this causes re- 
flections from undesired targets as well as a decrease 
in the energy falling on the desired target. This latter 
effect puts a low r er limit on the size of the target that 
can still be satisfactorily detected. Too small a beam- 
width causes the beam to be off the correct target 
over excessive periods because of the pitch and yaw 
of the airplane, and thus spoils the operation. This 
show's that the problem is definitely tied in with the 
stability of the airplane during the approach, as it is 
not worth while to use beams narrow- er than the aver- 
age value of the airplane deviations. 

The next problem is the choice of crossover for the 
beam. For a given beam there still is considerable 
leeway in the choice of the crossover point and, thus, 
in the angular sensitivity of the system. Crossovers 
at very low r power level result in loss of sensitivity 
because too little energy hits the target proper. Also 
there is chance of confusion because excessive 
amounts hit confusing targets. Crossover at peak 
pow r er, on the other hand, obviously results in com- 
plete loss of angular sensitivity. This problem too is 
tied in with the stability of the airplane during the 
approach, because excessive sensitivity results in 
off- target indications over too large a fraction of the 
time. Thus a study of the stability of the airplane 
during the approach, that is, of the apparent angular 
deviations of the target from the cross hairs both in 
time and magnitude, is important in the determina- 
tion of the optimum operating conditions. 

Finally, the best values of time constants in the 
system should be investigated. If taken over suffi- 
cient time, the average deviation of the target from 



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FUTURE DEVELOPMENTS 


245 


the optical cross hairs will be small because the pilot 
is constantly trying to fly his plane so that the target 
is centered. Therefore an increase in the time con- 
stant can, to a certain extent, overcome the troubles 
introduced by a very narrow beam or an excessive 
angular sensitivity. Long time constants, however, 
make the system sluggish; this is operationally un- 
satisfactory. 

As was pointed out, these factors are interrelated 
and the optimum set of conditions may therefore be 
expected to be a region rather than a mathematical 
point, and it may involve compromises. 

The existence of these factors was realized during 
the work on Vulture and Terry. At that time, how- 
ever, speed was so essential that existing units had to 
be used, even if it was well known that the beam- 
width was excessive. Plans for an investigation as 
outlined were made, but never carried out because 
of the lack of manpower. It is sincerely hoped that 
the armed forces will be able to give this problem the 
thorough analysis it certainly needs. 

20.7.3 New Combinations 

A review of the radar equipment in use at the end 
of World War II on fighter and fighter-bomber air- 
planes reveals a multiplicity of radar systems. There 
are, for instance, ARO, AN/APS-4, AN/APS-19, 
Falcon, Vulture, Terry, and several AI systems. The 
question arises whether these cannot be consolidated. 
Since the advent of the new radar techniques de- 
scribed in this chapter, such as the V presentation 
and the Terry system, possibilities indeed exist of 
combining all these functions in a single radar set of 
acceptable weight and size. The requirement of 
search demands a scanner that will scan over at least 
a sector of 60 degrees each side of dead ahead ; the re- 
quirement of ranging over land and that of blind fir- 
ing demand one that will produce a conical scan; 
those of AI demand a spiral scan. 

An antenna that will perform these functions can 
be obtained by small modifications of the AN/APS- 
19 scanner which has all but the conical scan. The 
latter can be obtained by arresting the nod, which 
produces the spiral, at the desired offset. 

The r-f, modulator, and receiver sections of such a 
system can be conventional, but range-tracking cir- 
cuits as described in this chapter have to be added 
and special attention given to the indicator. The 
search function can be presented in a B presentation 
or in a triangular presentation which will reduce dis- 


tortion of the shape of landmarks. The presentation 
for Vulture operation is as described above, except 
that the presentation will be turned 90 degrees so 
that range reads up, just as in the B presentation for 
search. For AI interception and for blind firing and 
approaches, the 2:1 frequency reduction in the multi- 
vibrator is changed to a 1:1 ratio. With proper phas- 
ing this results in a presentation as shown in Fig- 
ure 21. The vertical strips correspond to left-up-right- 
down positions of the scanner, and appearance of a 



Figure 21. V presentation for AI. 


target response in one of these quarters shows di- 
rectly which way the plane has to be moved to make 
it point at the target. Appearance of a response on a 
dividing line means that the target is, for instance, up 
and to the right. 

This interpretation is independent of the degree of 
offset of the scanner; therefore the presentation can 
be used for spiral-scan AI search, as well as for a fir- 
ing run or an approach on an airfield or beacon (or on 
isolated targets obscured by cloud layers) . It has the 
advantage of all panoramic presentations, that is, of 
showing returns from more than one target without 
upsetting the information from the desired one. As a 
result, ground clutter can easily be recognized as 
such. A further advantage is that no change in pres- 
entation is needed when going from search to attack. 
The possibility of using this presentation for blind 
approaches was demonstrated with a Vulture set in 
a number of approaches on a lighthouse in Boston 
harbor. Sperry was informed of the possibilities of 
this presentation for their AN/APS-19 radar, and 
made similar runs which confirmed the earlier results. 

The pilots would have to learn the interpretation 
of this presentation, but this should cause no trouble. 
In the investigation of the optimum parameters, de- 
scribed in Section 20.7.2, attention should be given to 
measurement of the angular deviation needed to pro- 


i 


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246 


COMBINED OPTICAL AND RADAR FIRE CONTROL 


duce a clear off-target indication in this system of 
presentation. Other presentations are possible, but 
this one seems to promise the greatest simplicity in 
equipment. 

The ARO and Terry tracking functions can be as 
described under the Terry system. One investigation 
was made and showed that an ASH system could be 
modified to do this job on a crash program basis 51 
but a development started from scratch would be far 
better. 

As a result of discussions with Navy personnel, the 
Navy decided to set up a new type number AN/APS- 
25 for such a set, and has prepared tentative speci- 
fications for it. It is hoped that this work will pro- 
ceed, as such a set should be perfectly possible and 
be usable for nearly all tactical problems. 

20.7.4 New Systems and Parts 

l/R Systems 

At least one computing gunsight which is in an ad- 
vanced state of development, the Draper-Davis or 
A-l sight, could be appreciably simplified if the radar 
set would put out l/R, the reciprocal of range, in- 
stead of range itself. Proposals have been made to 
obtain such a set by using the returned signal echo to 
retrigger the modulator. This would give a variable 
repetition rate, the recurrence frequency being pro- 
portional to l/R. At short ranges this would result in 
very high recurrence frequencies and thus in high 
duty cycles for the transmitter tube. At the same 
time, the power needed at short ranges would be low, 
and this condition does not appear to offer funda- 
mental difficulties. 

A system of this type would completely eliminate 
all the range-tracking circuits now used in the ARO 
set and in addition would eliminate the servo follow- 
up and the R to l/R conversion cam in the sight- 
computer. Some simple extensions would also permit 
the use of such a system for the operations now per- 
formed by the Terry system. 

No work has been done on this system beyond 
some preliminary rough calculations, but in view of 
the possible saving in weight and complexity, further 
work along this line seems desirable. 

Simplified Range Circuits 

Some circuits have been proposed 51 which might 
perform the functions of the ARO range-tracking 
system with considerably fewer tubes. They should 


receive careful consideration, particularly for use in 
a lightweight set such as the proposed AN/APS-25. 

Small Antennas 

The present conical-scan antennas, requiring a 
radome approximately 17 in. in diameter, still pre- 
sent a serious problem in installation and in drag. It 
is conceivable that jet-engined fighters such as the 
P-80 may find room in the nose to install the radar 
antenna. Some conventional planes may carry the 
radar in a detachable streamlined nacelle under the 
wings or the fuselage, although the latter may result 
in trouble from propeller modulation, caused by the 
propeller intercepting part of the radar beam and 
thus modulating the signals in a spurious fashion. 
In general there is a definite need for antennas that 
will have low drag and easy installation. Such an an- 
tenna may be made possible by the use of polystyrene 
rods. They can be shaped so as to have a paraboloid 
radiation pattern and yet have the advantage of be- 
ing long and narrow. Thus they have very little drag 
and can be installed in the leading edges of the wings. 
In order to obtain conical scan, such a polyrod an- 
tenna should be offset from the longitudinal axis and 
rotated around it. 

An antenna which switches lobes in elevation has 
been proposed for use with Vulture and Terry sys- 
tems in place of the conical scan. The fundamental 
difference in performance of the two antennas is that 
the lobe-switching antenna greatly decreases the 
modulation placed on the desired target’s echo by 
targets at the same range to the left and to the right 
of the desired target. The angular sensitivity of the 
Vulture system with conical scan is roughly 0.5 de- 
gree. Left-right targets more than 0.5 degree away 
from the desired target will introduce modulation on 
the desired signal echo. In a straight approach to a 
ground target, the total ground distance to left and 
right of the target over which unmodulated signal 
return would be obtained is less than 100 yd through- 
out the entire run. A lobe-switching antenna whose 
pattern and crossover are the same as the conical- 
scan antenna for the up-down positions would give an 
unmodulated return from left-right targets out to 
distances beyond half power for a range differential 
of less than 100 yd throughout the run. The effect of 
sloping or rough terrain and of banking of the air- 
craft is to reduce these side distances. Other effects 
are negligible. 

Preliminary investigations indicate that a lobe- 
switching antenna can be constructed of two pieces 


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FUTURE DEVELOPMENTS 


247 


of polystyrene or similar material shielded one from 
the other by an H-shaped reflector. The upper half 
will have a pattern corresponding to beam-up, the 
lower to beam-down. Power can be transmitted on 
both halves at the same time, but received power is 


switched from one to the other, so that a single re- 
ceiver will suffice. Satisfactory switches using modi- 
fied TR tubes have been developed. 27 ’ 31 At X band 
such an antenna would be about 7 in. long, 2 in. high 
and m - wide. 


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Chapter 21 

COMPUTER PROBLEMS 


21.1 THE ROLE OF RADAR IN 

AERIAL COMBAT 

21 . 1.1 Survey of Preradar Bomber 

Gunnery 

At the beginning of World War II, little attention 
was given to armament for bombardment aircraft. 
However, losses from fighter attacks soon forced at- 
tention to defensive measures. The British solution 
was based on night operation, whereas the AAF 
favored day operation in tight formations with 
fighter escorts. However, the addition of turrets and 
other gun positions became a “must ” in all bombers. 

The Lead Problem 

It soon became apparent that merely having guns 
was not enough. To hit an attacking fighter plane, 
except in a stern chase, requires careful aiming. If a 
bomber gunner at A (see Figure 1A) wishes to hit an 
attacking fighter then at B, he must aim so that his 
bullet goes in the direction AC where C is the place 
the fighter will be t seconds later, t being the time re- 
quired for the bullet to go from A to C. To achieve 
the bullet line AC it is necessary to point the gun 
along the line AD. The angle L between AB and AD 
is called the lead angle. Correct aiming requires that 
the gunner know the lead angle and also in what 
direction to lay off the lead angle. This means that 
he must be able to predict the future course of the 
attacking plane. It is obviously impossible to predict 
exactly what the fighter’s future course will be; the 
utmost that can be expected is a rough prediction 
based on extrapolation from the fighter’s course up 
to the time of firing. 

Position Firing and Vector Sights 

Numerous and widely divergent rules for gunners 
sprang up, but none of them were very effective until 
position firing was introduced. Indeed, the gunnery 
was so bad that in one bomber command only a 
negligible percentage of the gunners even knew the 
correct direction in which to lead. It is clearly im- 
possible to construct any set of practical firing rules 
that will work for all cases; however, if it is assumed 
that the attacking fighter is flying a pursuit course 

248 


then L (see Figure 1A) is given approximately by the 
formula (called the “own-speed lead” formula) 

T Bomber speed X sin 6 

I j = — 

Bullet speed 

and the direction of lead is toward the tail of the 
bomber in the so-called plane of action (the plane 
AEB in Figure 1A). 

The term pursuit course here refers to a course 
which enables continuous fire at the bomber and is 
more precisely described as an aerodynamic lead pur- 
suit course. The development of pursuit course theory 
started with the classical pursuit course in which the 
fighter is assumed to stay in a single geometric plane 
and point directly at a bomber flying straight and 
level with constant speed. First refinements led to 
the lead pursuit course. Aerodynamics are still ig- 
nored, the fighter is still assumed to fly with constant 
speed in a geometric plane but points his gun enough 
ahead of the bomber to secure hits; the next improve- 
ment was the aerodynamic pursuit course in which the 
fighter is assumed to fly so as to point always directly 
at the bomber, but the flight path is determined from 
aerodynamic considerations including such effects as 
angle of attack (mush) and changes in airspeed. The 
next refinement was to the aerodynamic lead pursuit 
course in the determination of which both aero- 
dynamic and ballistic considerations enter. For the 
theory of aerodynamic lead pursuit courses see 
reference 77. A companion report 78 contains numeri- 
cal data for a number of such courses. 

Note that in the above formula L depends only on 
the angle off (angle 6 in Figure 1 A) of the fighter and 
is independent of the range to the fighter; thus in 
using it the weakest point of preradar gunnery, 
namely, range estimation, is avoided. 

Position firing rules exploited the above formula 
for L; under them the gunner was instructed to lead 
the fighter in a direction toward the bomber’s tail 
and in an amount proportional to sin 6. More pre- 
cisely the range of values for 6 was split into several 
zones, an average value of L was determined for each 
zone, and the gunner was instructed to use this value 
throughout the zone. 

Aside from a few trimmings which are not dis- 
cussed here the vector sight was a mechanization of 
the lead formula used in position firing. It simplified 


KESTRICTED 


THE ROLE OF RADAR IN AERIAL COMBAT 


249 


B. BALLISTIC TRIANGLE 



A. LEAD DIAGRAM 

Figure 1 . Lead angle for a bomber under attack by a fighter (distances and angles not to scale). 


the gunner’s problem through automatically deflect- 
ing his line of sight by the appropriate position-firing 
lead. This reduced the gunner’s job to positioning 
his gun so that the target was centered in his sight 
and then pulling the trigger. 

All this simplicity was not without cost. The ap- 
proximation to L by the own-speed lead formula is 
rather rough even for pursuit attacks and is abso- 
lutely worthless in all other cases. For instance, if 
one bomber in a formation is being attacked by a 


fighter then only the gunners in the bomber under 
attack can hope to hit the fighter using position 
firing rules; for support fire from other bombers in the 
formation the formula for L is entirely different. In 
this connection, however, see a memo 80 giving rules 
for support fire with a vector sight. 

The whole problem of vector sights and position 
firing has been treated in considerable detail in publi- 
cations of the Applied Mathematics Panel and Sec- 


tion 7.2 NDRC. 8 - 67 - 6 


restricted' 


250 


COMPUTER PROBLEMS 


Rate Sights with Stadiametric Ranging 

Less rough formulas for L than those used for 
position firing rules or for vector sights give L as a 
function of R, a, a , e, e where R = range to target = 
AB in Figure 1A, a = target azimuth angle, e = tar- 
get elevation angle, and dots represent time deriva- 
tives. For still more precision additional derivatives 
are used. A very thorough analysis of the lead prob- 
lem has been made, giving various formulas for the 
lead. 63 - 88 The derivatives a and c can be computed by 
gyros or other rate measuring mechanisms on the 
assumption that the gunner tracks the target. 

Sights based on such formulas require much more 
from the gunner than a vector sight does. Tests have 
shown that although gunners can track fairly well, 
their estimates of range are very poor. Direct esti- 
mates of range in the air are likely to be in error as 
much as 100 per cent; the importance of a good 
range input can be judged from observing that L is, 
to a first approximation, proportional to R] so a 
given percentage error in range results in an equal 
percentage error in lead. 

Stadiametric ranging was one solution that was 
tried. For this the sight is provided with a reticle 
whose size is controlled by two dials. One of these 
dials is preset according to the wing span which the 
target airplane is known to have, and the other is 
adjusted continuously throughout an attack so as to 
keep the sight reticle the same size as the target 
image. This second dial is the range dial and feeds R 
(or some function such as log R) to the computer. 
The range dial may be controlled in many ways, in- 
cluding rotation of handlebar turret controls, foot- 
pedal controls, or a reversible motor drive controlled 
by a push button. Division 7 NDRC has sponsored 
extensive studies of the comparative merits of various 
ranging (and tracking) methods, and the British 
carried out similar (but less extensive) studies. 
Camera assessments of foot-pedal ranging by RAF 
fighter pilots resulted in claims of accuracy within 
10 per cent out to 600 yd, with a rapid decline in 
accuracy as ranges increased beyond 600 yd. Ameri- 
can experience gave less satisfactory results, errors of 
50 to 100 per cent being common at moderate ranges. 

21.1.2 Range-Only Radar for 
Bomber Gunnery 

A second solution to the ranging problem was the 
use of radar range, such as ARO described in Chap- 


ter 20 or AGS as described in Chapter 19. There were 
not many radar-range-plus-optical-sight installa- 
tions in bombardment aircraft during World War II. 
Some of the reasons for this lack of use are discussed 
in Section 21.3 and in Chapter 22. Our immediate 
concern is a comparison of stadiametric and radar 
ranging. Proponents of radar range have used the 
following arguments. 

1. Radar range does not depend upon target 
identification. This relieves the gunner of the neces- 
sity for setting in target wing span before an attack. 
Moreover, gunner errors in identification can lead to 
substantial errors in stadiametric ranging. 

2. Radar range is more accurate than stadiametric 
ranging (even assuming correct target wing-span 
setting) . In practice the gunner cannot give continu- 
ous attention to ranging and so will tend to approxi- 
mate true range by a step function. It is also claimed 
that even when giving full attention to ranging a 
gunner cannot get the accuracy of radar range. 

3. Radar can supply range information for much 
greater ranges than can be obtained stadiametrically. 
It is physically impossible for a gunner to range 
stadiametrically for ranges beyond the limit of 
camera measurements, which is approximately 1,000 
yd. But, whereas film assessments are made on the 
ground with unlimited time for precise measure- 
ments, stadiametric ranging during flight requires 
split-second attention, and so physiologically a gun- 
ner cannot be expected to match camera perform- 
ance. In contrast with this the present ARO (or 
AN/APG-5) is designed to measure ranges up to 
2,000 yd, and performance to far greater ranges can 
be expected from future systems. 

4. With the function of ranging removed from his 
duties, the gunner’s tracking should be better. 

So far as the author knows, no tests for the merits 
of the first claim have been carried out. The second 
and fourth claims have been examined under various 
test conditions, for example at Eglin Field and the 
University of New Mexico. The results 83> 94 substanti- 
ated (2) but failed to give much support to (4) . How- 
ever, no combat checks have been reported, and it is 
an important part of the argument in favor of auto- 
matic equipment in general that because of psycho- 
logical reactions a gunner’s performance under com- 
bat conditions may deteriorate considerably from 
that in training or under test conditions. The auto- 
matic equipment has no psychology and so will do as 
well in combat as in test. This contention has had 
ample verification in bombing where combat average 


, RESTRICTED 


THE ROLE OF RADAR IN AERIAI COMBAT 


251 


radial error ran as high as ten times practice radial 
error. 

21.1.3 Radar Tracking for Bomber 
Gunnery 

There is as yet no tracking radar for bomber gun- 
nery which is superior to visual tracking at its best. 
However, there are times when optical sighting is 
impossible and then there is no question of the need 
for radar tracking. The instances of combat at night, 
or in clouds are perhaps the most prominent ; another 
instance that came up during World War II was 
tracking through the vapor trail of a high-flying 
bomber. 

The function and some of the limitations of track- 
ing from a scope presentation of target position are 
discussed in Chapter 19. Most of the merits of AGS 
were related to wartime development conditions; it 
is not the kind of system one would choose to develop 
under peacetime conditions except possibly for tail- 
warning-only in aircraft where space and weight 
limitations preclude use of AGL. An AGS set is use- 
ful only when optical sighting is impossible and even 
then does not have the potentialities of a system 
which is fully automatic in its tracking operation. In 
view of these facts only fully automatic radar is in- 
cluded in the following comparison of radar tracking 
and optical tracking ; the discussion is also limited to 
cases of good visibility, since otherwise there is no 
competition. 

One important factor is the comparative accuracy 
of radar and optical tracking. Here the situation is 
greatly influenced by additional considerations, such 
as (1) the nature of the sight, (2) whether the track- 
ing requires positioning a heavy turret or merely a 
comparatively lightweight director sight, and (3) the 
angular rate of the target. No airborne radar system 
yet designed can establish the line to target at firing 
ranges as accurately or quickly as a trained operator 
handling a conveniently mounted lightweight di- 
rector sight (such as the GE standard B-29 pedestal 
sight would be if more conveniently mounted). If, 
however, the line of sight is controlled by positioning 
a heavy turret, the radar performance may well be 
fully as accurate as optical tracking. Furthermore, 
radar systems can begin tracking at ranges too great 
for optical sighting even under ideal conditions. 

Another factor is the importance of smooth track- 
ing. For some sighting systems, smoothness of track- 
ing is absolutely essential, whereas others are rather 


insensitive to roughness provided the line-of-sight 
error is kept small. This was borne out by tests at 
Austin, Texas, carried out by Division 7.2 NDRC on 
their testing machine. 17a (See Section 22.3.1.) Whereas 
the standard B-29 computer required smooth track- 
ing to build up leads, the Mk 18 seemed to function 
best when the absolute tracking error was minimized, 
regardless of roughness in tracking (provided the gyro 
did not tumble). Typical radar tracking has a high- 
frequency but low-amplitude “jitter”; future de- 
velopments promise to reduce this jitter sufficiently 
that smoothing with a very small time constant will 
give results acceptable for almost all computer re- 
quirements. 

Although radar tracking cannot (at least for the 
present) hope to surpass optical tracking at its best 
in accuracy and smoothness, it might turn out to be 
superior under combat conditions. A machine is not 
disturbed in its functioning by a bullet which “al- 
most hits it,” whereas some gunners might not 
track so accurately and smoothly under fire as in 
practice. 

One factor in which radar tracking has a natural 
advantage over optical tracking is in space require- 
ments. A radar antenna can be placed where a man 
could never place his eyes. Several radar antennas 
could give coverage in all directions and could be 
coordinated in such a way that a target which left the 
zone of coverage of one antenna could be auto- 
matically picked up by another antenna. Attempts to 
have B-29 gunners coordinate their efforts in this 
way met almost complete failure. The inherent 
superiority of machine over man is obvious in this 
case. 

21.1.4 Radar for Fighter Gunnery 

The lead for the fighter attacking a bomber on a 
pursuit course is almost the same as that needed by 
the bomber’s gunner in defending against that fighter. 
However, the fighter has no such easy solution as a 
vector sight. For instance, the leads given by a vector 
sight depend upon bomber speed and angle off of 
fighter, both of which are relatively accessible to a 
gunner in the bomber. But an attacking fighter pilot 
must guess at the bomber’s speed and estimate 6 
(Figure 1A) by comparing apparent length of the 
bomber’s wings and fuselage, or some equivalent 
feature which depends on angle off. Actually, some 
fighter pilots became so well versed in pursuit course 
tactics that they needed only to make certain of 


RESTRICTED < 


252 


COMPUTER PROBLEMS 


starting the attack at the correct position and could 
from then on fly by rote, just as a concert pianist 
plays without the score. Obviously, such performance 
cannot be expected of the average fighter pilot and 
so does not eliminate the need for computing 
sights. 

If the fighter plane has a rate sight with stadia- 
metric ranging, the pilot must again estimate 6 in 
order to range properly. For whereas stadiametric 
ranging for the bomber’s gunner involves framing i 
fighter approaching head on, the fighter pilot ap- 
proaching at a 60-degree angle off gets proper range 
by having the bomber wings fill just half of his 
reticle, and at 90 degrees must use fuselage length 
instead of wing span. A fighter pilot is burdened with 
far more duties in addition to shooting than is a 
bomber gunner, and so has less time to do all of these 
additional things necessary in stadiametric rang- 
ing. Some of the problems of the fighter pilot have 
been studied and summarized. 9S - 102 

These considerations lead to the conclusion that 
radar range is even more important in a fighter sight 
than in a bomber sight, at least so far as fighter- ver- 
sus-bomber combat is concerned. 

However, in World War II, far more AAF and 
Navy fighter combat was against enemy fighters 
than against enemy bombers. Fighter- versus-fighter 
combat brings in entirely different lead problems. 
The maneuverability of both target and attacker 
makes untenable many of the assumptions on which 
bomber gunsights are based. This tactical condition 
gave rise to doubts as to the advisability of installing 
even computing gunsights in fighters, much less radar 
range for these gunsights. As one naval officer put it, 
“Fighter combat in the Navy is largely a matter of 
pot shots following maneuvers for position in a dog 
fight.” Almost all fighter- versus-fighter kills were 
made from the tail cone. It is to be expected that 
future fighters will carry some kind of tail armament, 
and then there will be an undisputed need for com- 
puting gunsights and radar range. As the combat air- 
craft become faster, larger, and more complex in 
general, the distinction between fighter- versus-fighter 
and fighter-versus-bomber combats may become 
smaller, and overall protective armament will be 
needed in both fighter and bomber. 

The problems of the night fighter were treated in 
Part III and so have not been included in the present 
chapter, excepting in so far as the fire-control prob- 
lems of a turreted night fighter resemble those of a 
bomber. 


21.2 RADAR FOR AIR-TO-GROUND 
COMBAT 

As the war developed, the functions of fighter 
planes were greatly expanded, mostly in the direction 
of operations against ground targets. The most ob- 
vious instance of air-to-ground combat was strafing, 
a holdover from World War I, and a mode of attack 
which required no special equipment. It was found 
that bombs could be hung on a fighter plane ; indeed, 
the P-38 could carry almost as heavy a bomb load as 
a medium bomber, with the added advantage of being 
effective as a fighter as soon as the bombs were 
dropped. Many methods were devised for dropping 
bombs from fighters or fighter-bombers. These in- 
cluded toss bombing (discussed in Chapter 12), dive 
bombing, glide bombing, and trajectory bombing. 

21.2.1 Ballistic Radar Calibration 
for Cannon Fire 

In line with a trend toward heavier aircraft 
armament the B-25H and later the A-26 aircraft were 
designed to carry a 75-mm cannon. At first, the ac- 
curacy of cannon fire from an airplane was a dis- 
appointment to backers of the installation. It seemed 
likely that the AAF would drop the cannon-equipped 
aircraft, when the advent of Falcon (AN/APG-13A), 
a radar system designed to supply range from an air- 
plane to an isolated water target, changed the situ- 
ation. The sighting system used with Falcon is ex- 
tremely simple, although it requires a radar operator 
in addition to the pilot. This system is described in 
detail in the Falcon manual 33 and in somewhat less 
detail in a later report. 72 (See also Section 20.4.2.) 
The target appears to the radar operator as a pip on 
an M scope. He tracks the target in range by keeping 
a crank-controlled range-step in coincidence with the 
target pip. His crank is connected by a flexible shaft 
to a ballistic cam which positions the sight through 
which the pilot looks in aiming his aircraft at the 
target. 

The simplicity of the system is achieved only by 
limiting tactical conditions. Only one set of ballistic 
data can be put on the cam. This means that correct 
aiming is achieved only when a firing run begins at 
a specified range and altitude, with a specified air- 
speed and throttle setting. No account is taken of 
temperature effects; and the only provision for wind 
or target motion is a change in point of aim by the 
pilot. However, the aiming system proved to be very 
effective under the conditions for which it was de- 


RADAR FOR AIR-TO-GROUND COMBAT 


253 


signed. Some of the pilots were consistently able 
to score over 60 per cent hits on a simulated ship 
target. 122 

The limitations of the sighting system were 
brought into focus with the development of Vulture 
(AN/APG-13B), an air-to-ground ranging system 
not limited to water targets. This brought a new vari- 
able — target altitude — into the picture and pro- 
vided an impetus for a reconsideration of the whole 
aiming system, with particular reference to the possi- 
bility of making use of a “fudged ” range obtained by 
changing the calibration of the radar set. This adjust- 
ment, called ballistic calibration and the accompany- 
ing ballistic studies, are described in an Applied 
Mathematics Panel Report. 72 

The standard calibration of Falcon and Vulture 
consists of matching two variables: R, the slant range 
to target, and r, the range supplied to the cam. The 
mechanical and electrical connections between them 
guarantee that regardless of calibration r is a linear 
function (r = aR + 6) of R. The system can be cali- 
brated so as to achieve any value for a and b (within 
certain limits), the case a = 1, 6 = 0 representing 
standard calibration. A slide rule calculator, with 
scales for airspeed, temperature, and altitude is 
supplied to the radar operator for making the com- 
putations incidental to ballistic calibration. Specifi- 
cally, the slide rule computes n = 1,100a -f- 6 and 
r 5 = 5,100a + 6 as functions of airspeed, tempera- 
ture, and altitude. Ballistic calibration consists of 
matching the values r x and r 5 of r with the values 
1,100 and 5,100 of R. The Vulture-cannon combina- 
tion has a good test record 72 (see Chapter 20) , but 
was developed too late to see combat service. 

In spite of the good test record of ballistic calibra- 
tion it had some important limitations which in the 
author’s opinion preclude its use as more than an 
interim device. 

1. The pilot cannot read correct range to target 
from the range dial (adjacent to his sight). This is a 
definite handicap if he intends to follow the cannon 
firing by strafing, or if he is flying close to the ground 
and wishes to pull out at a given range. 

2. There is no provision for wind or target motion. 

3. It is necessary to decide tactics in advance, 
since the radar operator cannot very well calibrate 
during the firing run. This is perhaps one of the most 
serious limitations. 

4. There is only a two-parameter adjustment pro- 
vided to care for changes in correct lead arising from 
a large number of variables. By restricting the 


variety of attacks and neglecting some of the less im- 
portant variables it was possible to achieve much 
greater accuracy with ballistic calibration than had 
been hoped at first. However, even though use of 
ballistic calibration as compared to standard calibra- 
tion in some cases cuts down errors from 20 to 2 
mils, 72a the errors remaining in others are still large 
enough to condemn the device for anything more than 
interim use. The limitations could all be avoided by 
introduction of a computing unit whose inputs could 
be range (and perhaps also range rate) from Vulture, 
dive angle, altitude, airspeed, and temperature. 

21.2.2 Miscellaneous Information 
on Airborne Rocket Sight Settings 

Some of the major developments of World War II 
were in the field of rocketry. The early airborne 
rockets had such high dispersion that elaborate sight- 
ing systems for aiming were not worth while. As the 
dispersion was progressively reduced, the combat 
usefulness of the rockets increased and so did the 
demand for good sighting equipment. 

In considering the possible applications of radar 
ranging to rocket fire control, it is useful to have a 
rough overall view of the factors affecting the rocket 
fire. More complete discussions are to be found in 
publications of Division 3, NDRC, and of the Ap- 
plied Mathematics Panel. 1 ’ 3_7> 70 ’ 71> 112 

In the simplest case, where the correction for 
parallax due to the vertical distance between sight 
and rocket launcher is neglected, and where the 
launchers are assumed to be parallel to the zero sight 
line and the level line, the sight setting, following the 
theory developed at the California Institute of 
Technology [CIT], is given by (see Figure 2) 

S = M + fa 

where S is the sight setting in mils, M is the trajec- 
tory drop in mils, /is the rocket ballistic factor, 133 and 
a is the effective angle of attack of the level line, in 
mils. 

It should be noted that this quantity is not the 
same, in general, as the angle of attack as determined 
by wind-tunnel tests. From aerodynamic theory the 
angle of attack of any fixed line in the airplane can 
be represented in the form 

CW cos 8 

“ = — K 

where W is the weight (in pounds), 8 is the dive angle, 
Vi is indicated airspeed, and C and K are constants. 


RESTRICTED 


254 


COMPUTER PROBLEMS 


The airplane manufacturer supplies values of these 
constants. When these values were used in rocket 
fire it was found that errors of approximately con- 
stant value in mils occurred. This was taken care of 
in the CIT tables for rocket fire from various air- 
planes by giving new values to the constant K, 
leaving C unchanged. Thus one must speak of the 
value for a used in rocket fire as the effective angle 
of attack for rockets; correspondingly one should 
speak of an effective flight line. The whole theory is 
in an incomplete state. Some unpublished calcula- 
tions by the Applied Mathematics Panel and the 
Radiation Laboratory show that if the angle of at- 
tack term is large and if accurate long-range firing is 
desired, direct experimental determination of the 
whole fa term is probably necessary. 

When parallax is considered, a term 1,000 D/R mils 
must be added, where R is the range to the target and 
D is the vertical distance from sight to launcher, in 


yards; and if the launchers are set at an angle L 
mils above the level line (zero sight line) then the 
small, approximately constant, term (1 — f)L mils 
must be subtracted from the sight setting. 

The following tables give the variations in the two 
important terms, M and fa. The importance of the 
latter term arises from the fact that, unlike a shell, 
a fin-stabilized rocket tends to turn into the relative 
wind, following the line of flight rather than the line 
of the rocket launcher. Errors from any one source 
of less than 3 mils are regarded as negligible, with 
rockets as they now are made. 

Table 1 shows some values of the trajectory drop, 
M, and of the differences, AM, for 5-in. high velocity 
aircraft rockets [HVAR] and for 11.75-in. rockets 
(Tiny Tims) . These values are, of course, independent 
of the type of airplane. Table 1A gives the minimum 
trajectory drop for each rocket (reached when dive 
angle, true airspeed, and temperature are maximum) ; 


Table 1 . Effect of changes in range R, dive angle 8, true airspeed V, and temperature T, upon rocket trajectory drop M. 
Values of M and AM are in mils, based upon CIT tables. 



5!'0 HVAR 

11 '75 AR 


R (yd) 

500 

1,000 

2,000 

3,000 

4,000 

500 

1,000 

2,000 

3,000 

4,000 

A. Minimum M 


5 = 60° 

M 

8 

11 

18 

25 

33 

10 

15 

25 

35 

44 

AM for 1,000-yd intervals 

V = 400 k 

AM 



7 

7 

8 



10 

10 

9 



T = 100 F 












B. Maximum M 


8 = 0° 

M 

38 

50 

73 

99 

130 

46 

68 

107 

147 

190 

AM for 1,000-yd intervals 

V = 200 k 

AM 



23 

26 

31 



39 

40 

42 



T=0F 












C. Intermediate M 

o> 

II 

to 

o 

0 

M 

23 

32 

50 

70 

91 

29 

45 

73 

101 

130 

AM for 1,000-yd intervals 

V = 240 k 

AM 



18 

20 

21 



28 

28 

29 



T = 70 F 












D. Changes in 


8 = 0° 

M 

25 

35 

54 

77 

104 

31 

42 

69 

80 

103 

dive angle 

V = 240 k 


AM 

2 

3 

4 

7 

13 

5 

6 

6 

7 

8 

only. AM for 


8 = 20° 

M 

23 

32 

50 

70 

91 

26 

36 

54 

73 

95 

20° intervals 

T = 70 F 


AM 

5 

6 

11 

16 

21 

3 

4 

4 

3 

4 



II 

O 

o 

M 

18 

26 

39 

54 

70 

23 

32 

50 

70 

91 




AM 

6 

9 

14 

20 

26 

2 

2 

3 

3 

2 



8 = 60° 

M 

12 

17 

25 

34 

44 

21 

30 

47 

67 

89 

E. Changes in 


V = 200 k 

M 

26 

35 

54 

75 

98 

33 

50 

81 

111 

143 

true airspeed 

8 = 20° 


AM 

6 

7 

10 

14 

17 

8 

11 

18 

23 

30 

only. AM 


V = 300 k 

M 

20 

28 

44 

61 

81 

25 

39 

63 

88 

113 

for 100-k in- 

T = 70 F 


AM 

4 

5 

8 

10 

13 

6 

9 

13 

18 

22 

tervals 


V = 400 k 

M 

16 

23 

36 

51 

68 

19 

30 

50 

70 

91 

F. Changes in 


t = of 

M 

31 

42 

60 

80 

103 

37 

54 

84 

113 

142 

temperature 

5 = 20° 


AM 

5 

6 

6 

7 

8 

4 

5 

6 

7 

7 

only. AM for 


T = 40 F 

M 

26 

36 

54 

73 

95 

33 

49 

78 

106 

135 

miscellaneous 

V = 240 k 


AM 

3 

4 

4 

3 

4 

4 

4 

5 

5 


intervals 


T = 70 F 

M 

23 

32 

50 

70 

91 

29 

45 

73 

101 

130 




AM 

2 

2 

3 

3 

2 

1 

2 

1 

0 

0 



T = 100 F 

M 

21 

30 

47 

67 

89 

28 

43 

72 

101 

130 


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RADAR FOR AIR-TO-GROUND COMBAT 


255 


Table IB gives the maximum trajectory drop (mini- 
mum dive angle, true airspeed and temperature); 
Table 1C gives a representative intermediate case. 
Tables ID, IE, and IF show the way in which the 
trajectory drop varies when only one of the variables 
is allowed to change, the other two being held con- 
stant at the intermediate values. 

Tables 2 and 3 show the spread in values of the 


Table 2. Values of / and fa for a B-25 airplane 
(C = 125, K = 70 mils, w' = 33,000 lb) firing 5"0 
HVAR. 



T 

Indicated airspeed 

Vi in knots 



200 

240 

280 


OF 

0.83 

0.89 

0.93 

A. Values of / 

40 F 

0.78 

0.85 

0.90 


70 F 

0.76 

0.82 

0.86 


100 F 

0.72 

0.80 

0.85 

B. Values of a when 5=0° 

8 

-16 

-30 


OF 

7 

-14 

-28 

C. Values of fa 

40 F 

6 

-14 

-27 

when 5=0° 

70 F 

6 

-13 

-26 


100 F 

6 

-13 

-25 

D. Change in fa per 1,000-lb 
change in wt when 5=0° 

2 

2 

1 

E. Values of a when 5 = 20° 

3 

-19 

-33 


OF 

3 

-17 

-31 

F. Values of fa 

40 F 

2 

-16 

-30 

when 5 = 20° 

70 F 

2 

-16 

-28 


100 F 

2 

-15 

-28 

G. Change in fa per 1,000-lb 
change in wt when 5 = 20° 

1 

2 

1 


fa term for a heavy airplane (B-25) and for a light 
airplane (P-51), using 5-in. HVAR only. 

The effects of various kinds of errors are sum- 
marized below. 

Range Errors 

As will be seen from the table, an error of 100 yd in 
range produces an error of 1 to 4 mils in sight setting; 
this error is almost independent of range. 

Dive Angle Errors 

A change in dive angle produces a change in sight 
setting which is approximately proportional to range. 
At 3,000 yd, a 1-degree change in dive angle produces 
a change of 0.5 to 1 mil in sight setting for the 5-in. 
HVAR. 



Table 3. Values of / and fa for a P-51 airplane 
(C = 250, K = 24 mils, W = 9,500 lb) firing 5''0 
HVAR. 




Indicated airspeed Vi 

in knots 



240 

300 

360 

420 


OF 

0.84 

0.91 

0.95 

0.97 

A. Values of / 

40 F 

0.79 

0.88 

0.93 

0.95 


70 F 

0.77 

0.84 

0.89 

0.92 


100 F 

0.73 

0.83 

0.88 

0.91 

B. Values of a when 5=0° 

34 

13 

2 

-5 


OF 

29 

12 

2 

-5 

C. Values of fa 

40 F 

27 

11 

2 

-5 

when 5=0° 

70 F 

26 

11 

2 

-5 


100 F 

25 

11 

2 

-5 

D. Change in fa per 1,000-lb 
change in wt when 5=0° 

5 

3 

2 

2 

E. Values of a when 8 = 30° 

26 

8 

-2 

-8 


OF 

22 

7 

-2 

-8 

F. Values of fa 

40 F 

21 

7 

-2 

-8 

when 8 = 30° 

70 F 

20 

7 

-2 

-7 


100 F 

19 

7 

-2 

-7 

G. Change in fa per 1,000-lb 
change in wt when 8 = 30° 

4 

3 

2 

2 

H. Values of a when 5 = 60° 

5 

-5 

-11 

-15 


OF 

4 

-5 

-10 

-15 

J. Values of fa 

40 F 

4 

-4 

-10 

-14 

when 8 = 60° 

70 F 

4 

-4 

-10 

-14 


100 F 

4 

-4 

-10 

-14 

K. Change in fa per 1,000-lb 
change in wt when 8 = 60° 

2 

2 

1 

1 


Temperature Errors 

Changes produced by temperature variations are 
only slightly dependent upon range. On the average 
they amount to about 1-mil decrease per 10-degree 
rise in temperature, but there is a curious inversion 
in that for low temperatures the temperature cor- 
rection increases slightly with range, while at high 
temperatures it tends to decrease with range. 

Airspeed Errors 

A decrease in sight setting of from 5 to 15 mils is 
caused by a 100-knot increase in true airspeed. The 
change per knot is an increasing function of range. 

Altitude Errors 

No information is available on the effect of alti- 
tude. It is stated that the data are based on observa- 
tions at 2,500 ft above sea level. 

1HB 

w 


256 


COMPUTER PROBLEMS 



Changes in /« 

The rocket factor/ varies from 0.70 to 1.00. It de- 
creases with increasing temperature and increases 
with speed. For slower rockets it is very close to unity. 
The angle of attack 133 usually is not above 40 mils 
(for slow speed and heavy weight). It may become 
negative at high speeds and high dive angles. In con- 
sidering what the minimum value of a might be, it is 
clearly necessary to know the highest practical speed 
and dive angle. For example, with the B-25 a speed 
of 400 knots and dive angle of 60 degrees would give 
an angle of attack of —64 mils, but this situation is 
obviously impossible. The sample values of the fa 
term given in Tables 2 and 3 cover fairly wide ranges 
of conditions. 

21.2.3 Radar Ranging for Rockets 

One feature of the development of airborne rockets 
was a shift in requirements from airborne cannons to 
rockets. There were several good reasons for this 
shift. As compared with a cannon, rocket launchers 
have negligible weight. This made it possible to use 
rockets even on the lightest fighter planes. Rockets 
take up no space inside the fuselage. The explosive 
charge of a Holy Moses” (5 0 HVAR) is several 
times that of a 75-mm cannon shell. Rockets, unlike 
cannon, require no loading during flight. This makes 
it possible to fire rockets from a single-seater fighter 
plane. 

Although airborne rockets appear to be superior 
to airborne cannon for most purposes, there are cer- 
tain advantages of the latter which may save it from 


utter oblivion. In spite of improvements, rocket dis- 
persion remains at present two or three times as great 
as cannon dispersion, and even the fastest airborne 
rockets have only about two-thirds the velocity of 
the 75-mm cannon. These factors result in greater 
accuracy, smaller leads, and flatter trajectories for 
cannon than for rockets. There are tactical conditions 
under which these features of the airborne cannon 
make it the only effective weapon so far developed. 
One such condition is shooting into caves, where the 
flat trajectory and accuracy of the airborne cannon 
gave it superiority over all other weapons tested. 

For applications of radar to rocketry, aircraft fall 
into three classes. 

1 . Single-seater fighters. In these planes only auto- 
matic radar can be used for fire-control purposes. 
Space and weight are at a premium, so, unless a radar 
set makes very definite contributions, it will not be 
installed. 

2. Larger aircraft where no radar operator is avail- 
able. Radar installations for this class of aircraft are 
still limited to automatic systems. However, weight 
and size requirements are no longer so critical. 

3. Aircraft with a radar operator available. This 
does not necessarily mean full-time availability of a 
radar operator. With semiautomatic ranging, as pro- 
vided in AN/APG-13B (see Chapter 20), only oc- 
casional attention is required by the radar operator. 
(For instance, in the A-26 with 75-mm cannon, the 
radar operator was also cannoneer.) 

Although it is true that range-to-target is the main 
variable in determining correct lead for rocket fire, 


RESTRICTED 


RADAR FOR AIR-TO-GROUND COMBAT 


257 


it does not follow that a direct range input is neces- 
sary. The tardiness of development of an air-to- 
ground radar range system, together with the antici- 
pated weight and other requirements of radar, stimu- 
lated research on various substitutes for range. The 
number of rocket sights developed or proposed dur- 
ing the past two years is legion; it is not the author’s 
intention to discuss them here, aside from their 
methods of calculation of range. Discussion of many 
of these systems can be found in NDRC publications, 
especially those of Division 3, Section 7.2 and the 
Applied Mathematics Panel. 2 ’ 9> 10> 12> 13> 71> 101 > 104 > 138 

Several of the sights depend on a diving approach 
to the target ; R is then calculated geometrically. Re- 
ferring to Figure 2, we have R = h esc 8. Thus, a dive 
angle indicator and an altimeter can be used to cal- 
culate R. Now differentiate with respect to time, re- 
calling that dive angle is constant throughout an 
attack and get R = true airspeed = h esc 8 and 
thence R = Rh/h. In this equation h may be supplied 
by mechanically differentiating h or by observing the 
change in h during some short time interval. The 
actual sights do not usually explicitly obtain R; in- 
stead the full sighting formula may be written as a 
function of all variables finally used and mechanized 
as a whole. It should be mentioned that the quanti- 
ties used to compute range also enter in other parts 
of the sighting formula. 

Another class of sights made use of the fact that the 
curvature of a logarithmic spiral, at a point, is in- 
versely proportional to the distance from that point 
to the origin of the spiral. If an aircraft flies so that 
the angle 0 between the line of flight and the line of 
sight to the target is kept constant, then the path of 
the aircraft is a logarithmic spiral. Now if on any 
path the velocity of an aircraft is kept constant, the 
acceleration normal to the flight axis of the aircraft is 
proportional to the curvature of the flight path. Thus, 
if an accelerometer is placed so as to measure only 
the component of acceleration normal to the flight 
axis and if the pilot flies so as to keep the target cen- 
tered on a sight with constant offset (3, range can be 
computed. The sight offset can be either above or 
below the line of flight, although if it is above, control 
of the aircraft tends to become sluggish and may 
make tracking difficult. 98 

Several of the rocket sights developed for fighter 
planes were very compact and lightweight, some 
weighing under 30 lb. It is not hard to understand 
why installation of Terry (AN/APG-21), which 

weighed about 125 lb, was not regarded with favor 

. 


unless it could contribute to a sighting system which 
could do much more than these lightweight nonradar 
rocket sights. Some possibilities along such lines are 
discussed in the following section. 

However, for aircraft such as the B-25 or the A-26 
which are limited to fairly small dive angles, measure- 
ment of range from esc 8 becomes undependable. For 
this reason, the rocket sights developed for fighter 
planes were not suitable for use in larger planes. The 
development of Vulture made it important to design 
a rocket computer for medium bombers. In particu- 
lar, at the end of the war, the Radiation Laboratory 
was developing a rocket computer for use with Vul- 
ture in the B-25. This computer, known as the Vul- 
ture Rocket Computer, 29 was intended as an interim 
device, to serve only until more complete computers 
such as those discussed in the following section 
would be available. 

21.2.4 All-Purpose Sights 

A fighter aircraft is expected to perform a wide 
variety of functions, and each of these requires its 
own special equipment. Thus there are machine-gun 
sights, rocket sights, toss-bombing sights, torpedo 
directors, and cannon sights for fighters and for 
fighter bombers. The amount of space in the cockpit 
of a fighter plane does not permit separate installa- 
tions for each function. Common practice has been to 
decide which of the functions was primary, to install 
the equipment for that function, and then to look 
for ways to use this equipment for the remaining 
functions. The desirability of having a single sighting 
system which could handle all of the sighting func- 
tions of a fighter or of a fighter bomber became in- 
creasingly evident, especially with the increase of 
emphasis on airborne rockets. 

At least two all-purpose sights were under develop- 
ment by the end of World War II, the Draper-Da vis 
A-l sight 71a b and the pilot’s universal sighting system, 
[PUSS] . 12 < 13 

Because of uncertainties about the completion date 
of Terry both the A-l sight and PUSS had two lines 
of development, one based on both air-to-air and air- 
to-ground radar range and the other based on air-to- 
air radar range but with various substitutes for 
air-to-ground radar range. So far as the trajectory- 
drop terms in the rocket leads are concerned, substi- 
tutes for radar range may be reasonably satisfactory. 
However, such substitutes do not allow for the leads 
due to target motion or wind. It is obviously highly 
desirable to have a sighting system which is effective 


RESTRICTED 


258 


COMPUTER PROBLEMS 


in rough weather and against moving targets (such as 
tanks); this can be achieved if air-to-ground radar 
range is included in the sighting system. The fact that 
Terry can serve for both air-to-ground and air-to-air 
ranging is essential in its application to all-purpose 
sights. 

21.3 COORDINATION OF RADAR AND 
GUNSIGHTS 

21.3.1 General Discussion of 
Gunsights 

This section is written to supply background about 
gunsights. It is not intended to treat any specific gun- 
sight or theory of gunsights in detail. This has been 
done in publications of the Applied Mathematics 
Panel, of Section 7.2, NDRC, of the Army and Navy, 
and of the computer manufacturers. The following 
set of references is limited to the first two sources and 
make no pretense of completeness. 8> 12> 13 - 16 • 59 ’ 61 -w. 

74-76, 88, 108, 111, 114 

The simplest sights are ring and post or optical ring 
sights which are noncomputing. A slightly more com- 
plicated sight is the vector or own-speed sight which 
was discussed in Section 21.1.1. Obviously, none of 
these sights has any requirement for either radar 
range or radar tracking. 

Next in order of complexity are the rate sights. 
Such a sight computes the lead L as the solution of a 
differential equation such as 

( — a)uL + L = ua — (3 (1) 

where u = time of flight in seconds of bullet to pres- 
ent position (from A to B in Figure 1A), 

< t — angular velocity of the line of sight in 
radians per second, 

a = a parameter of the sight; in practice a is 
between 0 and — 1, 

0 = ballistic deflection (caused by bullet slow- 
down in flight). 

If the gunner is tracking the target, a = 0 (see Figure 
1A) and the term ua is the so-called “angular-travel 
lead, ” the lead L can be regarded as an exponentially 
smoothed value of the angular-travel lead plus the 
ballistic deflection with smoothing time constant 
k = ( — a)u . 58a 

The mechanism to measure a may be a mechanical 
or electrical integrator (as in the Sperry K-4 family 
and the Fairchild K-8) or a gyroscope (as in the 
Mk 18). 


The non-gyro sights are unable to distinguish be- 
tween motion of the gun platform and motion of the 
sight head. Consequently pitch, roll, and yaw of the 
aircraft result in the feeding of false information to 
the sight. Gyro sights avoid this difficulty, provided 
the gunner continues to track the target accurately 
while his plane’s course is unsteady. This is apt to be 
a considerable chore, so the most recent sights have 
been stabilized, i.e., have automatic compensation 
for deviations of aircraft from a rectilinear path. 

The computation for a may be made in components 
or all at once. The Mk 18 is a single (free) gyro sight 
which computes a directly. The B-29 standard com- 
puter is a two-gyro sight which computes in com- 
ponents. The gyro is used entirely differently in these 
two cases. In the Mk 18, the gyro has a gimbal 
mounting and is free to precess in whatever direction 
the forces acting on it require. The amount of pre- 
cession is controlled magnetically, and is proportional 
to the lead set up. This is accomplished by attaching 
to the gyro a mirror whose deflection determines the 
position of the sight reticle. In contrast with this in 
the B-29 standard computer each gyro has a single 
pivotal mount and is thereby constrained to motion 
in one direction (relative to its case). The amount 
of motion in this permitted direction is very small, 
just enough to make a contact which activates a force 
just sufficient to neutralize the precession force of the 
gyro. The neutralizing force is measured and thus 
transmits information from the gyro to the computer. 

When <t is computed in two components care must be taken 
to avoid errors due to the phenomenon of “gun roll.” The 
mathematical basis of gun-roll errors is the fact that a rotation 
in space is determined by three parameters (for instance 
azimuth and elevation of the axis of rotation plus a third 
parameter giving the amount of rotation) and measurement 
of angular motion in two components must neglect one of these 
parameters. If the sighting system is constructed so as to 
measure the right pair of parameters no error is introduced, 
but otherwise gun-roll errors are present. A detailed analysis 
of gun-roll errors is given in Applied Mathematics Panel 
Studies. 63a> 88 

Lead-computing gunsights can be divided into two 
classes according to whether the gunner has direct 
control of the sight line or of the gun line. In a 
director system the gunner has immediate control of 
the line of sight and as he tracks the target the com- 
puter determines the lead and positions the guns. 
The standard B-29 computer is a director system. 
In a disturbed-reticle sight, the gunner has immedi- 
ate control of the gun line. The sight case is mounted 
on the gun and the computer positions the line of 


COORDINATION OF RADAR AND GUNSIGHTS 


259 


sight with respect to the gun. The Mk-18, K-3, K-8, 
and A-l are examples of disturbed-reticle sights. 
There are computing sights which are neither direc- 
tors nor disturbed-reticle sights but lie somewhere 
between. (For instance in the S-8b, the gunner has 
direct control of the lead angle rather than either 
sight line or gun line.) 88d These intermediate types 
were not developed in time for use in World War II 
but seem likely to have considerable future use. 

A director sight is evidently operationally simpler 
for the gunner, but it requires much more machinery 
with consequent addition of weight, size, and main- 
tenance problems. 

In general, director systems are designed for re- 
motely controlled gun turrets, whereas disturbed- 
reticle sights are used in local turrets. 

Problems of operational stability have arisen in 
the use of tracking radars as components of airborne 
fire-control systems. Some discussion of these prob- 
lems appears in Chapters 18 and 19, but much re- 
mains to be learned in this field. Some preliminary 
work along these lines has been done. 88, 89> 139> 140 


21.3.2 Calibration of Computers 
and the Role of Range Rate 

In actual practice a formula for L such as given by 
equation (1) may be improved if u is replaced by 
some other quantity v. Since u equals range divided 
by average bullet velocity, it is primarily a function 
of R. Replace u by v = v(R) in (1) and solve for v, 
obtaining 


I ±+A. 

a dL 


(2) 


Now, for any given attack course C, the right- 
hand side of (2) can be computed giving v as a func- 
tion v(R, C) of range and of the particular course. If 
several courses Ci, C 2 , • • • are considered we can 
compare the corresponding functions v(R, Ci). If the 
courses chosen are representative, and if some esti- 
mate is made of relative importance of these courses, 
then a weighted average 


v(R) = 


Wjv(R,Ci) 

Wi 


of-flight calibration. There have been a number of 
other ways proposed for determining v(R), many of 
which involve considerable refinements and improve- 
ments over the simple one described above. In par- 
ticular, the use of time-of-flight calibration is not 
limited to sights which mechanize equation (1) ; it can 
be used with any lead-computing sight. In all cases 
the essence of the process of time-of-flight calibra- 
tion is to regard the time-of-flight setting v(R) as a 
function of range whose form is chosen directly with 
a view to the overall performance of the instrument, 
rather than to fit ballistic tables. 

This point of view was emphasized by Professor 
Draper first in his antiaircraft sights and later in his 
aircraft sights. Draper’s work has been carried on and 
extended by I. Kaplansky and other members of the 
Applied Mathematics Panel. 79> 88, 95, 103, 110 

Some radar systems (in particular f-m sets) give 
both R and R (range rate) as outputs. The possible 
use of R in lead-computing sights was the subject of 
considerable discussion during the latter part of the 
war. It is obvious that allowing v to be a function 
v(R, R, C) allows the effective inclusion of more 
courses Ci in averaging, this time obtaining an 
average function v(R, R) to be mechanized. 

In particular, it has been suggested that use of both 
R and R as inputs would allow calibration to fit both 
pursuit courses and straight line courses. But it has 
also been argued that use of R without also using 
angular accelerations would not be of much advan- 
tage. The author’s opinion is that the lead-computing 
sights developed by the end of World War II were 
not sufficiently accurate to justify refinements such 
as R. This is not a reflection on the sight designers, 
since they were faced with the problem of quickly 
getting something into production which was better 
than vector sights or position firing rules, and in this 
they seem to have been successful. However, post- 
war developments of lead-computing sights should 
be based on careful studies of the value of range rate 
and of angular acceleration inputs, keeping in mind, 
of course, possible limitations to the accuracies with 
which these quantities may be measurable in prac- 
tice. Attention is called in particular to Hestenes’ 
proposal 61 for a vector-rate sight in which inputs of 
range rate and angular acceleration are included. 110 


of the functions v{R, Ci) may be far superior to the 
original time of flight u{R). 

The above process for obtaining a function v(R) to 
use instead of u in equation (1) is an example of time- 


21.3.3 Problems in Coordination 

One of the early decisions of NDRC was to set up 
one division for fire control and another for radar. 


260 


COMPUTER PROBLEMS 


Section 7.2, NDRC, had responsibility for airborne 
fire control and Division 9 of the Radiation Labora- 
tory, a contractor of Division 14, NDRC, had re- 
sponsibility for airborne radar of all kinds. The 
liaison between these two agencies was intermittent 
in character although it improved during the last 
year of the war, too late to have any influence aside 
from that on postwar developments. The Army and 
Navy had somewhat similar divisions of responsi- 
bility in their agencies. There were gunsight develop- 
ments under direct Service contracts entirely unre- 
lated to Section 7.2, NDRC, and there were airborne 
radar fire-control systems under direct Service con- 
tracts, entirely independent of the Radiation Labora- 
tory. The net result was a chaotic condition in which 
there was no single agency with overall responsibility 
for airborne fire-control systems and this condition 
was reflected in the lack of progress in getting effec- 
tive airborne fire-control systems into combat. Per- 
haps the best way to illustrate the general coordina- 
tion problem is to treat a few special cases. 

In designing the ARO system, it was necessary to 
decide what form the range presentation should take. 
A scope presentation requires operator interpreta- 
tion. Since it was considered essential to have an 
automatic range signal, only electrical or mechanical 
outputs were considered. 

After a series of conferences it was decided late in 
1942 to standardize the ARO output as a direct-cur- 
rent voltage proportional to range. However, the first 
contracts let were for ARO units to feed range into 
the Sperry K-4 sight, and as a result of this the ARO 
units actually manufactured were equipped with gear 
boxes which converted the output voltage to a shaft 
rotation proportional to range, one complete revolu- 
tion for each 100 yards range. Further standardiza- 
tion conferences were held throughout 1943, 1944, 
and 1945, until finally in the spring of 1945 complete 
specifications for ARO were agreed upon. Most of the 
modifications agreed upon in this series of confer- 
ences were engineering changes to improve perform- 
ance but some of them had definite influence on 
methods of using ARO with gunsights. The sight 
makers paid almost no attention to these modifica- 
tions, partly because they were not directly informed 
and partly because they were not enough interested 
in being informed. Some of them felt that they 
should not “waste time” on radar until it was com- 
pletely standardized. 

There were exceptions to this general indifference 
to ARO. For instance, the K-4 plus ARO combina- 


tion was carefully nurtured, but unfortunately the 
K-4 sight was obsolete by the time ARO was ready for 
use. The Draper-Davis A-l sight was designed from 
the outset to accept radar range in the same way as 
the K-4. Performance of the ARO A-l system during 
tests at Eglin Field in 1945 was one of the bright 
spots in the airborne fire-control program. 

Meanwhile other lead-computing sights had been 
developed without reference to possible use of radar 
range. The Mark 18 and the General Electric stand- 
ard B-29 computer were both designed to accept log 
R rather than R, the former for ranges up to 800 
yards, the latter for ranges up to 1,250 yards (ARO 
is designed to supply range up to 2,000 yards). This 
difference in maximum range together with other 
differences makes it impossible to find any single 
range output mechanism which would fit both of 
these sights. 

When radar range was required for those sights, 
sight maker, radar maker and other interested 
parties all sat down together for a series of confer- 
ences to decide who should be responsible for the 
intermediate gear boxes or other gadgets needed to 
convert from radar output range function to sight 
input range function. 

In the case of the B-29 computer, the radar system 
used for range was AGS rather than ARO, but this 
changed nothing so far as combination of sight and 
radar range was concerned. The first few installa- 
tions made use of a mechanical gear box, but in later 
production the computer was modified to accept the 
standard output of voltage proportional to range 
and the combat wing described in Chapter 19 was so 
equipped. The marriage of radar and computer was 
considerably simplified by the fact that in this case 
sight manufacturer was also radar manufacturer. 

The case of the Mark 18 was much more difficult, 
because of the fact that the sight required a shaft 
rotation, whereas in the B-29 computer either shaft 
rotation or voltage was possible. The sight maker 
finally accepted responsibility for gear box modifica- 
tions to convert straight range to logarithmic range 
and a few gear boxes were available in time for 
equipping several planes for test purposes and ten 
planes for combat tests. (These aircraft saw no aerial 
combat because of reduced enemy fighter action. 
However, the equipment had a good maintenance 
record in the combat theater.) 

The role of radar in rocketry was discussed above. 
So far as interim rocket sights for fighters were con- 
cerned, the nonuse of radar was fully justified. How- 


RESTRICTED 


COORDINATION OF RADAR AND GUNSIGHTS 


261 


ever, rockets might have been used on medium 
bombers much sooner than they were if the liaison 
between sight designer and radar designer had been 
closer. On the one hand, the radar experts were not 
aware of the potential value of rockets; while on the 
other hand the sight experts were so wrapped up in 
the fighter plane problem, where radar range was 
not then feasible, that they did not visualize that 
radar already developed might be useful in other 
cases. The sighting methods which they had de- 
veloped for fighter planes broke down for the less 
maneuverable medium bomber. The sight experts 
were reluctant to leave the fighter field where sights 
were needed in thousands to start afresh in the 
medium bomber field where the total number of 
sights needed might never have exceeded one thou- 
sand. This exclusive emphasis on a major problem 
resulting in a complete neglect of minor, though still 
important, problems is in itself a good instance of 
lack of overall direction of the airborne fire-control 
program. 

Another place where lack of overall direction 
showed up clearly was in the AGL system programs. 
The early AGL’s, AN/APG-1 and AN/APG-2, were 
too heavy and bulky for use in bombers. (One small 
squadron of P-61’s equipped with AN/APG-1 and a 
director type sighting system were headed toward 
combat at the end of the war.) 

The later AGL’s were considerably lighter. How- 
ever, the development of AN/APG-3, which was 
intended, among other things, to replace AN/APG-15 
for the tail defense of the B-29, was delayed for at 
least a year because of installation problems. The 
radar set was still too large to fit in the space then 
available and the aircraft manufacturer was not 
enthusiastic about redesigning the tail turret to ac- 
commodate the radar. 

21.3.4 Possible Solutions for the 
Coordination Problem 

There are a number of reasons why coordination 
problems of the kind described above may be ex- 
pected to become more severe in the future. The 
number of functions of an aircraft has been increas- 
ing and each new function means new coordination 
problems. It will not be easy to install anything at all 
outside the fuselage of the newer very high speed 
aircraft. Accuracy criteria will become much more 
severe in the atomic era, since missing even a few 


enemy planes or missiles might spell destruction for 
whole cities. 

These factors prompt the suggestion that in future 
developments of the combat airplane, the aircraft 
manufacturer should have overall responsibility for 
every piece of equipment his plane is to carry. He 
would, of course, be at liberty to subcontract, but 
he would have responsibility for fitting all of the 
pieces together, from the drawing board stage on. 
He would be encouraged to draw upon resources of 
government-sponsored research organizations, and 
would naturally be expected to meet certain Army- 
Navy requirements. 

The aircraft manufacturer might wish to subcon- 
tract for all of the fire-control equipment on his air- 
plane. Suppose, for instance, he wished to subcontract 
for the tail turret of a bomber, complete with arma- 
ment, search, and gunlaying equipment. He would 
then have to design his aircraft with due considera- 
tion to all features of tail-turret operation, including, 
for instance, provision of ample power supply to the 
turret and all of its equipment, provision for radar 
antenna installation, allowing for aerodynamic drag 
of the tail turret and equipment attached outside it. 
The turret contractor might, in turn, wish to sub- 
contract the gunlaying system. If he gave separate 
subcontracts for radar and gunsight he would en- 
counter some problems similar to those already dis- 
cussed; unlike those situations, there would be no 
lack of authority to make the decisions necessary in 
solving these problems. 

It is clear that the Service agency which placed the 
contract would have to maintain close contact with 
the manufacturer, to see that the latter did not 
merely choose the system which would cause him the 
least trouble, or which was the most attractive for 
purely business reasons. 

A change from present policy less drastic than 
assigning prime responsibility to the aircraft manu- 
facturer would be to concentrate complete responsi- 
bility in some single Army-Navy agency, so that dis- 
cussion of engineering details would not have to be 
carried on at so many different points and with such 
frequently changing personnel as was usual during 
World War II. This agency, which would have to 
possess a high degree of technical competency, and 
be in close touch with development agencies, would 
contract separately for the components (aircraft in- 
cluded), with carefully written specifications to re- 
duce problems in coordination. 


RESTRICTED 


Chapter 22 

ASSESSMENT PROBLEMS 


Experience throughout World War II showed that 
assessment of fire-control systems is not easy. The 
reports of the Applied Mathematics Panel and of the 
various Applied Mathematics Groups, particularly 
at Columbia and Northwestern Universities, as well 
as those of Section 7.2 of NDRC, give extensive dis- 
cussions of airborne fire-control systems and of the 
problems encountered in evaluating them. 12 - 13 - 88 - 90 - 117 

From these agencies came such outstanding de- 
velopments as the Texas sight-testing machine 14> 15< 
u, is, 86 an( j the “tricamera” method for the assess- 
ment of flexible gunnery. The latter was worked out 
jointly by the Army (Eglin Field), Navy (Patuxent) 
and NDRC. 73 - 121 (See Section 22.3.1.) 

This chapter deals with the assessment of radar 
fire-control systems. However, the radar portion can- 
not be planned or tested separately from the sighting 
system into which it is incorporated. Accordingly, 
this discussion will follow rather general lines. 

22.1 SPECIAL NATURE OF ASSESS- 
MENT OF AIRBORNE FIRE-CONTROL 

EQUIPMENT 

22.1.1 Contrast between Airborne 

and Land-Based Testing 

In the assessment of fire-control systems on the 
ground, the problem is simply that of shooting at tar- 
gets similar to those encountered in combat. Com- 
parative target scores then give an accurate measure 
of the relative merits of the various fire-control de- 
vices. Even when the system is mounted on a ship, it 
is possible to compensate adequately for the motion 
of the platform. 

Difficulties arise when the target is airborne, but 
when the fire-control system itself is carried in an 
airplane the problem of assessment is enormously 
more complex. In the air one is firing from a gun plat- 
form which is in motion. This motion may be uniform 
or accelerated. Changes in motion may be smooth or 
abrupt. The changes in motion suffered during com- 
bat will depend to a considerable degree upon tactical 
considerations which change from time to time. The 
strain of combat may reduce the accuracy of the 
operator. The pilot will usually crowd on all possible 
speed during combat, while under test conditions he 


will fly more slowly for the sake of safety, accuracy, 
and airplane maintenance. Furthermore, the kine- 
matic problem presented to the computer varies with 
the target motion. When a slow-flying towed target 
is used as a substitute for a fast, rapidly accelerating 
fighter airplane, the results will differ enormously. 
The effects of these variables will of course be differ- 
ent for the different situations: air-to-air or air-to- 
ground fire; fixed or flexible gunnery; 0.50-caliber 
ammunition, cannon or rocket projectiles. 

For these and other reasons, the target scores ob- 
tained in firing tests may have very little relation to 
the results which would be obtained in combat. The 
possibility of a fire-control system actually perform- 
ing better in combat than in firing tests is indicated 
by a consideration of the Applied Mathematics Panel 
hit probability nomogram (Figure 1). 

The total system (sight, gun, tracking operator or 
mechanism, ranging operator or mechanism) will 
have two types of errors: (1) bias error, that is, an 
error inherent in the sight, of constant magnitude and 
direction, and (2) dispersion or random error, com- 
posed of gun and ammunition dispersion, tracking or 
aiming error and ranging error. The probability of 
obtaining a hit upon a target of given area in square 
mils depends upon the bias error and the standard 
deviation of the dispersion, in such a way that when 
the bias error is held constant, the hit probability 
passes through a maximum value as the dispersion 
increases from zero. This maximum is reached when 
the standard deviation of the dispersion equals the 
bias error. 

As an example, consider a sight for which parallel 
and perpendicular components of the errors are 
equal. Suppose that there is a bias error of 10 mils in 
the sight mechanism. By rotating a straightedge 
about the 10-mil mark on the left-hand scale of 
Figure 1, it is seen that the hit probability is very 
small for low values of the standard deviation of the 
dispersion (e.g. 0.02 per cent when S = 4 mils), that 
it increases up to a value of S = 10, at which it is 
about 0.5 per cent, and then decreases for greater 
values of S. 

An actual example of this was believed to have 
been found in the tests of the K-15 or Mark 18 gyro 
sight. 94 The behavior of the system using radar range 
information supplied by AN/APG-5 was compared 


262 


RESTRICTED 


ASSESSMENT OF AIRBORNE FIRE-CONTROL EQUIPMENT 


263 


with that using manual stadiametric ranging. The 
sight had a bias error of 16 to 23 mils. The mean 
deviation of the radar range data was much smaller 
than that of the manual ranging (19 yd compared 
to 115 yd in one instance). Assessment showed that 
the hit probability with the inaccurate manual rang- 
ing was equal to or greater than that with radar 
range. 

In terms of the nomogram, this means that the 
total dispersion with radar ranging was so small that 
the maximum hit probability had not yet been 
reached, while with manual ranging the dispersion 
was presumably so large that the maximum had 
been reached or exceeded. 

Now consider that the gunner’s tracking becomes 
gradually worse (as it might in combat) ; imagine that 
all errors excepting tracking errors remain constant. 
The resulting increase in total dispersion will steadily 
decrease the hit probability with manual ranging; but 
with radar ranging it will cause an increase at first, 
followed eventually by a decrease. The hit proba- 
bility therefore will soon become and will then always 
remain greater for radar than for manual ranging. 

The above arguments, of course, represent an ex- 
trapolation and simplification to an extent that may 
not be justified in the actual case under discussion. 
However, they illustrate the problems which can 
arise, and serve to emphasize the need for coordina- 
tion of effort in all phases of the development of a 
fire-control system. 

22.1.2 Importance of Photographic 
Analysis 

In air-to-air fire it is impossible for a test to simu- 
late combat conditions by shooting real bullets at a 
real airplane. If real bullets are replaced by frangible 
bullets, a procedure of great usefulness in training, 85 
the results are unreliable for assessment purposes be- 
cause of the great difference in ballistics. The substi- 
tution of towed targets for real airplanes results in 
oversimplification (see Sections 22.3.1 and 22.3.2), 
the seriousness of which depends upon circumstances. 

Even if realistic conditions could be had, there 
would be no way of learning whether a miss was 
caused by error in aim or in the sighting system. 
Finally, realistic shooting of any kind is statistically 
inefficient, in that the misses (usually a large fraction 
of the rounds fired) contribute little to the assess- 
ment. It is true that a radio device was under de- 
velopment at Laredo, Texas, which would record the 


-26 


1-10 


H - EXPECTED NUMBER OF HITS. 

N = NUMBER OF R0UNDS = I00. 

A r AREA OF TARGET IN SQUARE 
MILS ( 8 SQ FT AT 300 YO S ). 

S„ 2 = 0f| 2 + 0b || 2 = TOTAL VARI- 
3 ANCE IN SQUARE MILS OF 
PARALLEL (TO PLANE OF ACTION) 
COMPONENTS OF FLUCTUATING 
AND OF QUASI- STEADY ERRORS. 


--3 


r 21 


r 19 


S l = °Q1 = TOTAL VARI- 
ANCE IN SQUARE MILS OF 
PERPENDICULAR (TO PLANE OF 
ACTION) COMPONENTS OF 
FLUCTUATING AND OFQUASI- 
STEADY ERRORS. 


30 -q 


29” 


* ■■ * PARALLEL COMPONENT 
OF TOTAL BIAS OF MEAN POINT 
2 OF IMPACT. 28 ~ 

«*. i = PERPENDICULAR COMPONENT 
OF TOTAL BIAS OF MEAN 
POINT OF IMPACT. 


25T 



DIRECTIONS 

TO DETERMINE H.GIVEN << || ,.C X ,S || . ) AND S ± ; 

I. ALIGN*,, AND S„ WITH A STRAIGHT-EDGE AND MARK THE 
CORRESPONDING POINT P, ON THE RIGHT-HAND VERTICAL SCALE. 

2 ALIGN*. AND Si AND MARK THE CORRESPONDING POINT P a 
ON THE LEFT HAND VERTICAL SCALE. 

3. ALIGN P, AND Pa. AND READ THE VALUE OF H ON THE CENTRAL 
VERTICAL SCALE 


12 - 


11^ 


Figure 1. Hit probability nomogram constructed by 
the Applied Mathematics Panel. 

nil 


Hit probability H = 


NA 
2inS 1 1 St 


e 


2S 2 

il 


25^ 


RESTRICTED , ff 


l.nilni. Ii.nl m , IimiIh ii In ill in. 1 . 1 1 1 1 ,. .1 l.i i ■ 1 1 i 1 1 In ■ i 1 


264 


ASSESSMENT PROBLEMS 


distance and direction of bullets passing a towed 
target, but it was not used in World War II. 

When the bullets are replaced by camera shots, 
however, everything but the firing can be simulated, 
and from statistical theory the effects of the shooting 
can be superposed. 84 These effects include among 
other things ammunition dispersion and “ gun climb.” 
The nature of the latter in aerial gunnery was ap- 
parently not well understood in the earlier part of 
World War II. Studies of it were made by the Eighth 
Air Force 127 and at Laredo. 128 ’ 129 

Recording devices are also needed for the motion 
of the airplane and other pertinent data. For this pur- 
pose, the camera is probably the most reliable instru- 
ment, although mechanical drum recorders were some- 
times used. Temperature extremes and centrifugal 
accelerations such as occur in airplanes cause trouble 
in the usual instrument of this type. In any case, the 
recording of numerical data in an airplane by a 
human observer is highly unreliable. 


The testing agency should have first-class military 
maintenance personnel and equipment, and should 
carry out a definite program of preliminary check- 
ing on the bench, in the airplane both on the ground 
and in the air, as well as continued routine preflight 
checks. The development agency should make sure 
that adequate instructions for such checks are in the 
possession of the testing agency, and should maintain 
close contact with it throughout the tests. 

Tests have been held up for considerable periods 
by such elementary difficulties as the sticking of a 
servo-motor, water in the antenna, or oil leaking into 
a cable. Occasionally a piece of equipment, such as a 
sight, was tested for more than twice its prescribed 
useful lifetime. 

Failure of maintenance during the acceptance tests 
was one of the causes referred to in Chapter 17 for 
the extreme delay in the introduction of airborne 
radar fire-control equipment. 

Assessment of Maintenance 


22.1.3 Special Relation of Main- 
tenance and Assessment 

The remarks in this section might apply to many 
phases of military development, but the problems 
here described were accentuated by the peculiarities 
of airborne equipment and by the nature of radar. 

Maintenance during Testing 


One of the prices which may have to be paid for a 
new development is an increase in maintenance re- 
quirements. It is an important function of the test- 
ing agency to determine what this price is, so that 
policy-determining agencies will have enough infor- 
mation to make an intelligent decision. To this end, a 
complete record both of routine and of extraordinary 
maintenance should be kept throughout the test. 


Adequate care of a new system during the testing 
procedure is obviously necessary, but in practice is 
very hard to secure. Even first-class maintenance per- 
sonnel normally do not understand the scientific 
basis of a new device well enough to be able to handle 
it without some special training. This difficulty was 
enhanced during World War II by the policy of as- 
signing low priority to the needs of the testing 
agencies, so that there were never enough trained 
maintenance men to carry on the routine preflight 
checks, let alone to handle serious maintenance prob- 
lems. Adequate test equipment and maintenance 
facilities were also lacking at the testing agencies. 

The natural response of the development agency, 
such as the Radiation Laboratory, to this condition 
is to send its own representatives to maintain the 
equipment during testing. However, this practice has 
certain limitations, for such representatives are not 
typical maintenance men and erroneous impressions 
may be gained as to the serviceability of the equip- 
ment. 


22.2 TESTS ON A RADAR FIRE- 
CONTROL SYSTEM — AGS 

One of the systems described in this part 
(AN/APG-15B, Chapter 19) was tested in some de- 
tail by the Radiation Laboratory at Bedford Field. 
In these tests many of the major problems of assess- 
ment were well illustrated. Accordingly they will be 
outlined here and used as a particular example, be- 
fore the more general discussion of assessment of 
fire-control systems. Recommendations of a general 
nature will, however, be included here also. A com- 
plete account of the results of these tests is given in 
reference 28. 


22.2.1 Test Design 

A thoroughly worked out program for such a test 
is essential. For some simple device a test program 


RESTRICTED 


TESTS ON A RADAR FIRE-CONTROL SYSTEM - AGS 


265 


may ask only the question, “Is it serviceable?” and 
the test will answer yes or no. One of the mistakes 
encountered during World War II was the attempt 
to apply this oversimplified criterion to such an 
elaborate mechanism as a fire-control system. Of 
course general serviceability must be insured, but 
those who are planning to use the system need far 
more detailed information about its performance. 
They need it in quantitative form. They need 
numerical values for such quantities as the proba- 
bility of securing a hit or destroying the target under 
definite conditions. Sufficient data must be collected 
so that the averages will mean something, and the 
data must be treated according to sound statistical 
methods. The Latin square method of laying out 
experiments should be employed where much infor- 
mation must be extracted from limited data. 142 ’ 143 
However, where it is possible, the collection of data 
should be liberal, so that statistically significant re- 
sults can be obtained without extremely laborious 
calculation. Probable errors must be calculated, for 
in practice they may be just as important as the 
quantities themselves. 

The test of the AGS system was formulated along 
the following lines. 

Questions to be Answered 

As described in Chapter 19, the AGS system pro- 
vides range information to the standard B-29 com- 
puter for daytime use, and in addition, for conditions 
of restricted visibility provides a spot on an indicator. 
This spot is centered if the antenna is being aimed at 
the target. A thorough assessment of these functions 
requires answers to the following questions. 

1. What is the accuracy of the range information? 

2. When the gunner is tracking perfectly, that is, 
keeping the indicator spot centered, does the gun 
actually point at the target? 

3. Is the distance of the spot from the center of the 
indicator directly proportional to the angular devi- 
ation of the target? 

4. How much does the indicator spot lag behind 
the pointing of the antenna assembly? 

Subsidiary problems were presented by the per- 
formance at very high altitude and at low altitude 
over water. These were not actually studied, because 
of the end of World War II. The second phase of the 
test was also designed to secure information about 
the performance of the gunner as affected by the func- 
tioning of the computer. 

REST1 


Stations and Operators Required 

The equipment was installed in a TB-24 airplane, 
and an FM-2 was used as the target. As the location 
of the gunner’s scope makes photography difficult, a 
duplicate scope was located in the waist. The gunner 
acted as flight supervisor, giving orders to the pilots 
of the two airplanes and to the camera operator. He 
loaded the gun camera, and of course did the actual 
tracking. 

Another person, stationed at the second scope, was 
responsible for the operation of the radar and com- 
munications systems. He loaded the scope and com- 
puter cameras and adjusted the cross hairs on the 
scope. 

The camera operator turned the cameras on and 
off on signal from the gunner. He filled out the report 
form and reported to the gunner when all was ready 
for a pass. He timed the use of film with a stop watch, 
not allowing more than 100 sec per 50-foot maga- 
zine. If more than one pass was made per magazine, 
he marked the division between passes by pressing 
down the button to operate the code marker. He was 
responsible for securing a boresighting shot of 50 
frames with the gun camera, on the ground before 
each flight. 

Program of Missions 

Range Accuracy. Four missions, ten passes each, 
closing rates 25, 50, 75, and 100 mph, altitude 6,000 
to 10,000 ft. 

Pointing Error. Two missions, one with computer 
in, one with computer out, starting range 800 to 
1,000 yd, five passes with target airplane doing 
weaving S turns, five passes with 30-degree crossover 
course, crossover at 400 yd. 

Scope Linearity. Two missions, with different indi- 
cator-amplifiers, range 800 to 1,000 yd, target air- 
plane flying straight and level directly behind the 
bomber, altitude 6,000 to 10,000 ft, measurements 
with sighting angles of 2, 4, and 6 degrees at 3, 6, 9, 
and 12 o’clock directions on the scope. 

Lag between Turret and Indicator Spot. One mission, 
range 800 to 1,000 yd, altitude 6,000 to 10,000 ft, 
target airplane flying straight and level, back and 
forth slewing of sight at amplitudes of +4 and ±2 
degrees, and at three rates — slow, medium, and fast; 
medium rate corresponding approximately to average 
aim-wander period. Slew both in azimuth and eleva- 
tion; also slewing ±30 degrees across target; also 
miscellaneous slewing as in typically bad tracking. 



266 


ASSESSMENT PROBLEMS 


22.2.2 Instrumentation of AGS 
Tests 

Cameras 

Three standard GSAP cameras, N4A, 24 volts, 
modified to use Wollensak 17 mm lens, were used. 
The gun camera was provided with a 3-in. lens to 
give greater range accuracy, and a red filter to insure 
contrast. The boresighting procedure consisted of 
mounting a boresight telescope on the gun, pointing 
it at some reference object and taking about 50 
frames with the gun camera. This gave the refer- 
ence point from which all target angles were meas- 
ured. 

The computer camera photographed the range dial 
on the computer. The zero and slope set were to be 
checked before each flight. A third camera photo- 
graphed the remote scope in the waist. 

Synchronizing Mechanism for Cameras 

While the speed of GSAP cameras can be regu- 
lated, it is impossible to synchronize two of them 
perfectly; one will gain one or two frames per hundred 
on the other. In order to be able to match the simul- 
taneous frames, a camera control and coding box had 
been developed. The switch for turning the cameras 
on and off was located on this box, and cables from it 
to the cameras provided the 24- v power for operating 
them. As described below, the cables also provided 
accurately timed pulses of electric energy which 
actuated the magnets that pulled out the over-run 
indicators, thus coding the pictures. 

The mechanism for providing these pulses con- 
sisted of a speed-governed motor which operated a 
small gear box and thence a shaft on which a cam was 
mounted. This cam was in contact with a microswitch 
during part of each rotation of the shaft; during this 
time the microswitch was closed. This caused elec- 
tric pulses of a fraction of a second in duration to be 
sent to the magnets in the cameras which in turn 
removed the over-run indicators. The box was also 
provided with a button for closing the over-run 
indicator circuits manually, thus allowing the indi- 
cators to remain out of the fields of the cameras 
continuously for any desired length of time. This 
form of manual code marking was used to denote the 
beginning or end of a run. 

Just before the end of the war, a more satisfactory 
synchronizing mechanism was developed by the 
General Electric Company and modified by the Radi- 
ation Laboratory. 46 This consisted of a pair of sole- 


noids to be inserted in the GSAP camera, which 
could actuate the shutter on receiving impulses from 
a cam-operated source at speeds up to 4 or 5 per sec. 
In this way perfect synchronization was obtained, 
without the inconvenience caused by differing 
camera speeds, and the magazine of film lasted much 
longer. When the GSAP cameras were run in normal 
fashion, at 16 frames per second, every fourth or 
fifth frame was analyzed. This solenoid drive allowed 
the cameras to run at exactly the desired speed. In 
this way a magazine of film lasted four or five times 
as long, thus avoiding the large waste of film, and 
more important, allowing longer missions to be flown 
in situations where reloading in the air was not pos- 
sible. 

Target Airplane 

Since accurate measurement of target width is 
essential, the FM-2 was at first provided with wing- 
tip lights. However, the airplane power supply proved 
inadequate for lights bright enough to be practical. 
(At Eglin Field a separate generator was used to 
supply lights of several thousand watts power.) Visi- 
bility up to about 1,000 yd was secured by painting 
black bands on the wings. These bands had to be 
extended all the way around the leading edge of the 
wing in order to be visible under all conditions. 
Accurate measurement of the distance between the 
stripes to 0.1 ft was important, as the overall 
accuracy of the range determination depended upon 
it. Four persons each measured the distance twice 
and the mean was taken. 

Because of the great difficulty of measuring air-to- 
air range accurately by photographic means at ranges 
beyond 1,000 yd, and because of the high degree of 
precision shown by the radar range system, it is sug- 
gested that an airborne range-only radar set (such 
as AN/APG-5) be used for the measurements of 
target distances in future tests. It should be noted 
that this would be reliable with present systems only 
if three or four photographic checks were made in 
each pass at shorter ranges, and if the set were ac- 
curately calibrated, especially for the slope setting. 

22.2.3 Analysis Methods 

Precautions to Insure Analyzable Data 

It is not uncommon to find after a mission has been 
flown that part or all of the resulting film is useless 
for purposes of analysis. Probably the most important 
precaution is to process and view the film very 


RESTRICTED 


TESTS ON A RADAR FIRE-CONTROL SYSTEM — AGS 


267 


quickly, so that trouble will be detected in time to 
avoid repetition of the error on another mission. 

The GSAP camera is not a very satisfactory in- 
strument, and must be checked frequently. Refilled 
magazines often stick. Before a mission the films 
may be marked with a pencil through the openings 
in the magazine, and the cameras operated very 
briefly to see whether the pencil marks disappear. If 
the over-run indicator has a dark background in the 
field of view it may be invisible. This should be 
checked with a standard camera boresight tool, and 
a small white card should be mounted so that the 
indicator can be seen clearly against it. 

None of the actual operations of the mission should 
be left to the pilot of the airplane. He is occupied 
with flying and should not be required to push but- 
tons, guess at range, or perform similar functions. 

Frequent boresighting shots are necessary, as it is 
easy for the camera to be knocked or shaken out of 
alignment. Range calibration shots should be taken 
at various ranges, and in no case should this impor- 
tant function be replaced by a mere calculation from 
the focal lengths of the camera and projector, as 
camera lenses of the same specifications may differ 
by 10 per cent or more. 

Adequate labeling of films, ordinary precautions 
to insure good pictures, and adequate forms for re- 
cording data are needed. It is highly desirable that 
the person in charge of the group which later will 
analyze the data should himself observe typical test 
missions, and a representative of this group should 
be present on all missions. 

Errors in Optical Range Determination 

The following discussion is based on a report 43 of 
the fire-control analysis section of the Radiation 
Laboratory. 

The determination of the distance R from the gun 
to the target by optical means can be accomplished 
by projecting the photograph of the target on a 
screen, measuring the width of the image of the tar- 
get and calculating the range from the formula 



w 


where T is the actual size of the target (assuming that 
it is approaching head on, i.e., the aspect angle is 
zero), w is the measured size of the picture of the tar- 
get, and & is a constant, the value of which depends 
upon the units employed, the focal lengths of the 
camera and the projector, and the distance between 
the projector and screen. Computation of k from 


these quantities, Lowever, is difficult and inaccurate, 
a fact which is perhaps not widely enough appreci- 
ated. In an assessment report 81 appearing after the 
end of World War II, the surprising statement is 
made that a simple proportion gives range to an 
estimated accuracy of 3 per cent “regardless of the 
length of the range” ! It is true that the context im- 
plies that maximum ranges of about 1,500 yd were 
being considered, but even below this limit the error 
is an increasing function of range. It is better to make 
a number of calibration shots at a series of known 
ranges of an object with known dimensions, obtaining 
a good value of k by averaging. Alternatively, the 
angle S subtended by the object can be measured by 
a surveyor’s transit at the camera. Then, 

R = ttT cot ^ 

2 2 

if R and T are in the same units. 

Since 

R _ k 
T~w’ 

then 



if R and T are in the same units. Since S is usually 
less than 10 degrees, cot S/2 can be replaced by the 
value of 2/S in radians. 

The most important single error in this range de- 
termination is that involved in measuring w on the 
screen. Since the error in w depends upon the ac- 
curacy with which the two ends of the calipers can 
be placed upon the image, it will be about the same 
for small images as for large images. Now the width 
of a given target image on the screen is inversely 
proportional to the range. Therefore the percentage 
error in range, from this source, is directly propor- 
tional to the range; or the absolute error in range, 
expressed for example in yards, is proportional to 
the square of the range. To illustrate, suppose that 
the image can be read to 0.02 in. and at a range of 
500 yd appears 2 in. wide. The error will be 1 per 
cent, or 5 yd. At 1,000 yd the image will be 1 in. 
wide, giving an error of 0.02/1 = 2 per cent or 20 yd. 

The other important error, that in focusing the 
image on the screen, seems to be proportional to w. 

Table 1 shows the combination of these errors 
(overall percentage error) in range for different sizes 
of target at different ranges, assuming a measuring 
error of 0.02 in., a focusing error of 1 per cent, and a 
projection distance of 80 in. 


RESTRICTED 


268 


ASSESSMENT PROBLEMS 


Table 1. Per cent error in range. Blanks correspond to values of the subtended angle greater 
than the field of view of the camera lens. 


Range R 


Target size T in feet 


111 J £U uo 

25 

50 

75 

100 

200 

300 

400 

500 

1,000 

500 

1.49 

1.18 

1.11 

1.09 






1,000 

2.36 

1.49 

1.26 

1.18 

1.09 

1.06 




2,000 

4.34 

2.36 

1.76 

1.49 

1.18 

1.11 

1.09 

1.08 


3,000 

6.41 

3.33 

2.36 

1.90 

1.32 

1.18 

1.13 

1.10 


4,000 

8.50 

4.34 

3.00 

2.36 

1.49 

1.26 

1.18 

1.14 

1.08 

5,000 

10.59 

5.38 

3.67 

2.85 

1.69 

1.37 

1.24 

1.18 

1.09 

6,000 

12.70 

6.41 

4.34 

3.33 

1.90 

1.49 

1.32 

1.23 

1.10 


By considering this table one sees that for accurate 
measurement of ranges greater than 1,000 yd, it is 
desirable to have a target divided into measured 
portions (such as a bridge or a long building with 
definite divisions between sections), so that several 
portions can be measured at long ranges, and one 
portion at short ranges. However in air-to-air tests 
this is impossible, so that a practical limitation of 
about 1,000 yd is imposed upon photographic rang- 
ing. Radar ranging is, however, good to far greater 
distances. 

An error in range will be produced if the line from 
the camera to the target is not perpendicular to the 
measured line in the target, that is, if the aspect 
angle is not zero. If the angle is known, a correction 
can be made, which is negligible for angles up to 
5 degrees. With larger aspect angles, such as are en- 
countered in assessing fighter gunnery, a small un- 
certainty in aspect angle will produce a considerable 
error in the range. Further discussion of this point, 
and tables of errors, are given in the report cited. 45 

Some Results of AGS Tests 28 

Some samples of the range accuracy measurements 
are given here to illustrate the kind of accuracy 
which was obtainable. Figure 2 shows typical photo- 
graphs of the target airplane which were used for 
photographic range determination. Figure 3 gives 
sample curves comparing radar range with photo- 
graphic range at four different closing rates. Figure 4 
shows such a comparison in one of the early runs of 
the test, and is given to show how bad the results 
can be if improper attention is given to the various 
precautions. For example, the distance between the 
stripes on the target airplane had been measured 
only once, and there was some uncertainty about 
the calibration of the radar set; other details also 
had not been standardized. 


Throughout the test, after the correct procedure 
had been established, the radar range agreed with the 
photographic range well within the probable error of 
the latter. 

Figure 5 shows the results of one of the tests of the 
lag of the spot indication behind the turret. In this 
test the radar system had a time constant of 0.1 sec, 
while under the usual operating conditions a value 
of 0.25 sec was used. 

The term time constant as used here refers to the time 
constant of the chief portion of the circuit as calculated from 
the resistance and capacity (Jc = CR; k will be expressed in 
seconds if C is in microfarads and R in megohms). 

If a linear differential equation of the form 
d\ 

fc-+X = 0 
at 

governs the displacement X of the spot on the scope, then 
X = X 0 e an( j jc w iu ^ the time required for the spot to 
move a fraction 1 — 1/e of the distance which the target moves 
in a sudden displacement from rest. 

If the target is moving back and forth with respect to the 
radar antenna in some periodic fashion, for example as a sine 
function, then 

d\ 

k (- X = sin cot 

dt 

and 

X = (1 + k- co 2 ) ~ * sin (cot — tan~ l kco) + ce ~^ k 

The time lag k' of the spot is seen to be tan -1 ku/oo, which 
is less than k. 2S 

Referring to Figure 5, where k = 0.1 sec, it is seen 
by comparing the peaks of the two curves that k' is 
about 0.13 sec. With k = 0.25 sec, k' is found to be 
0.3 sec or greater. The fact that these time lags are 
greater rather than less than the time constant cal- 
culated from the circuits shows that a linear differ- 
ential equation of the type discussed is only a rough 
approximation. Greater time lags than these can be 
tolerated before the tracking system becomes un- 
stable in the hands of a gunner. 


RESTRICTED r 


ASSESSMENT OF FIRE-CONTROL SYSTEMS 


269 



c 

Figure 2. A. Target airplane at 1,060 yd. B. Target airplane at 270 yd. C. Target airplane at 200 yd. 


22.3 ASSESSMENT OF FIRE-CONTROL 
SYSTEMS 

The preceding discussion is based upon the direct 
experience of the analysis group at the Radiation 
Laboratory. This group also took part in or followed 
many other tests of fire-control systems. The follow- 
ing discussion is based upon these observations. 
More detailed accounts will be found in the reports 
of the Applied Mathematics Panel, NDRC Section 
7.2, the AAF Proving Ground at Eglin Field (usually 
issued as AAF Board reports), the Navy testing 
agencies at Patuxent (Naval Air Station, Bureau of 
Aeronautics) and Inyokern (Naval Ordnance Test 
Station, Bureau of Ordnance), as well as other Army 
and Navy testing agencies. 12 - 15 - 17 - 18 ’ 62 ’ 84 ’ 86 - 88 ’ 90 ’ 92 ’ 96 ’ 116 


Airborne fire-control systems fall into three groups : 
control of flexible gunnery, as used in the turrets of 
bombers (and to a limited extent in night fighters); 
control of fixed gunnery in fighters; and air-to- 
ground fire control for cannon and rockets. This 
chapter will not deal with the analytical assessment 
of sights, that is, the separate calculation of the 
errors which would be shown by a mechanically per- 
fect sight perfectly operated. 62 - 96 

22.3.1 Assessment of Bomber 
Fire-Control Systems 

The purpose of this assessment is to determine the 
effectiveness of the system in shooting down or driv- 
ing off attacking fighters. 


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■A 


270 


ASSESSMENT PROBLEMS 



Towed Targets and Drones 

Towed targets have been used to test the fire of 
bomber turrets, but the results of these tests have 
little value, for the behavior of the targets is very 
different from that of attacking fighters. 

A better approximation to fighter aircraft is given 
by drones, i.e., remotely controlled small-scale air- 
planes. Extensive testing with these is, however, 
extremely expensive for the relatively small amount 
of information which they provide. 

Photographic Methods 
Camera assessment of flexible gunnery has proven 
to be the most satisfactory method because, every- 
thing considered, it is the most realistic. It allows 
the use of real fighter airplanes as targets and it re- 
cords the fate of all of the bullets that would be fired 
in an attack, rather than only those few that hit the 
target. A comparison of camera tests against a towed 


target with firing tests against the same towed target 
was made at Eglin Field and a close correlation of 
scores was found . 121 

In all types of photographic assessment a camera 
is mounted with its axis parallel to the gun bore and 
wired so that in a simulated attack the camera oper- 
ates when the trigger is pressed. These gun-camera 
photographs, properly calibrated, show the angle 
by which the gun is leading the target. An evaluation 
of the lead which should have been used 58 requires 
a knowledge of the course of the target airplane. This 
in turn requires a stable frame of reference with 
respect to which the motion of the target can be 
measured. A method of photographing the target 
against a background of clouds or mountains was 
developed at the University of New Mexico. 93, 94, 119 
It is useful for rapid assessment, but the so-called 
tricamera method and the closely related deflec- 
tometer method 60 - 73 - 82 - 84 - 91 - "• 10 °* 106 - 118 - 121 used at 


ASSESSMENT OF FIRE-CONTROL SYSTEMS 


271 



Figure 4. Graph of radar range obtained under poor operating conditions. 


Eglin Field and Patuxent, while requiring slow and 
painstaking work, are capable of higher accuracy. In 
the former, a group of three cameras, the tricamera, 
mounted rigidly together, photographs the complete 
field of vision to one side of the bomber, and includes 
a wing tip and the tip of the stabilizer fin in the 
photographs. If the bomber is flying absolutely 
straight and level, the successive pictures of the tar- 
get in this field give the necessary data; but if the 
bomber is subject to motion about its longitudinal, 
lateral or vertical axis (roll, pitch, or yaw, respec- 
tively) then the apparent motions will be in error. 
These motions can be compensated by a calculation 
based upon a knowledge of the amount of roll, pitch, 
and yaw, which in turn can be obtained from syn- 
chronous photographs of an attitude gyro. The de- 
flectometer is an instrument which measures the 
position of the gun in the bomber frame of reference 
by recording the turret position. This is done by 


photographing either the turret machinery itself, or 
dials operated remotely by selsyn motors. 

A valuable sight assessing machine was developed 
at the University of Texas 12_15, 17, 18 by means of 
which an operator in the laboratory can track the 
projected image of an airplane with an actual sight, 
the path of this image being determined by a cam 
which in turn is made to duplicate an actual combat 
course. The operator’s platform can be made to 
undergo roll and yaw. From the photographic records 
obtained by this machine, it is possible to compare 
the lead put in by the sight with the correct lead, 
either assuming perfect ranging or assuming some 
form of incorrect ranging as supplied by a cam, or 
allowing stadiametric ranging by the operator. The 
advantage of this machine is that it computes the 
gun pointing errors very accurately and quickly. It 
seems probable that an increasing amount of confi- 
dence will be placed in it, although the test is less 


restricted.. 


272 


ASSESSMENT PROBLEMS 



slewed periodically. 


realistic than the camera assessment in actual 
flight. 

For the sake of completeness, two ideas should be 
mentioned which have been proposed as still more 
accurate assessment methods. These are the air-mass 
coordinate method, 11 - 115 and the photometeoronic 
method. 88 - 130 The latter has attained a high degree of 
precision in other applications at Aberdeen. 

22.3.2 Assessment of Fighter 
Gunnery 

Towed Targets 

The use of towed panel targets for assessing the 
performance of a fighter fire-control system is not 
objectionable in principle (although here also the 
shots which miss serve very little purpose), for the 
towed target can be made to approximate the mo- 
tion of a bomber reasonably well, except for speed. 
The following difficulties were encountered in World 
War II. Although these are small in themselves, any 
of them can cause the test to break down. 

1. If the fire-control system is good, many targets 
will be shot away and lost unless a multiple line 
(similar to parachute shrouding) is used. 

2. The small size and nonreflecting character of 
the panel target as compared to a bomber requires 
that a corner reflector be inserted in it if radar rang- 
ing or tracking is used. Even optical ranging presents 
a minor difficulty, as the standard reticle is too large . 

3. Rounds fired against lost targets must be sub- 
tracted in calculating scores. 

4. Fighter pilots may be very far off in their esti- 
mates of range, e.g., 300 yd called 1,000 yd, so that 
some instrumental (camera or radar) record of range 
is needed when the shooting accuracy of a system is 


being measured in different range “ blocks” or re- 
gions. 

For a really accurate appraisal of the system in 
terms of probability of effective hits, a knowledge of 
the vulnerable area of the bomber at different aspect 
angles would be needed. 113 In the assessment of 
bomber gunnery it was assumed that the vulnerable 
area of a fighter was a 5-ft circle. This was reasonable, 
as the fighter would normally be flying almost di- 
rectly toward the bomber. However, it is unlikely 
that a similar procedure would be satisfactory for 
the bomber. 

Photographic Methods 

Camera assessment of fighter sights is less highly 
developed at present than that of bombers, and 
special difficulties present themselves because of the 
more complex motions of the gun platform in space 
and because of the pattern of gun fire. Two of these 
motions are “mush ” (the effect of the angle of attack 
of the fixed gun, which varies with speed and curva- 
ture of the path), and “skid’ ’ or side-slip. The former 
can be measured roughly and taken into account. 
The latter can be detected (although not always by 
the usual indicator), and those runs discarded in 
which it is present (see Section 22.3.3). 

The basic problem in this assessment can be 
stated in terms of the photograph of the target 
bomber taken by the fighter gun camera. Assuming 
that the center is the boresight point, then the point 
must be found where the bullet will be when it has 
traveled a distance equal to the range (strictly, 
future range). This requires an estimate of range, 
gravity drop, mushing, and skid effects. This point 
must then be compared with the point where the 
bomber will be at that time. The motion of the 


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ASSESSMENT OF FIRE-CONTROL SYSTEMS 


27.3 


bomber therefore must be known. Three methods 
have been proposed for finding this. The one which 
was developed the furthest was that of photograph- 
ing the fighter from the target bomber. This was 
used at Patuxent. 81 Another method, involving only 
films taken from the fighter, was developed at Eglin 
Field. In it, a plastic disk with a pin passing through 
the center was mounted upon a movable pedestal. 
The pin represented the bomber. By rotating this 
model until its shadow coincided suitably with the 
projected image of the bomber on a screen one could 
determine azimuth and elevation of the bomber, 
angle off and range. This method was a refinement of 
the W-2 fighter-sight assessor for training fighter 
gunners, used at Foster Field. 125 By the end of the 
war no publication on it had appeared from Eglin 
Field, and its potential accuracy had not been de- 
termined, although one well-trained operator was 
able to reproduce his readings on angle off to within 
1 degree. 

A third device was designed by S. Eilenberg and 
J. H. Lewis, 105 based upon a proposal of R. Hayward, 
Mount Wilson Observatory. It is similar in principle 
to the W-2 method but uses a small luminescent 
model which is compared with the film by a viewer 
directly, instead of using projection on a screen. 

From the information obtained by one or more of 
these methods the direction of flight of the bomber 
is determined, the kinematic lead is calculated and 
measured off on the photograph and the spot thus 
found is compared with the calculated location of the 
bullet. 

22.3.3 Assessment of Air-to-Ground 
Fire-Control Systems 

The two preceding sections dealt largely with 0.50- 
caliber fire. In air-to-ground combat this is used only 
for close-range strafing; fire-control problems here 
are concerned with airborne cannon and rocket fire, 
where the use of radar increases the practical range 
up to 6,000 yd, as discussed in Chapter 20. 72, 121 

In the assessment of range accuracy it was pos- 
sible to make use of the principle of a target com- 
posed of measured sections (Section 22.2.3, “Photo- 
graphic Methods”), so that reasonable photographic 
range accuracy was obtained at 6,000 yd. The most 
effective sectional target was one at Dugway, Utah, 
consisting of a square array of roads, 2,000 yd on a 
side, with parallel crossroads at 200-yd intervals. 
The aiming point was a corner reflector located at a 


road junction. Photographs at long range showed ten 
200-yd segments, and at the end of a run showed 
only one segment. Analysis of these films was carried 
out rapidly and accurately, and without much of the 
usual strain on the analysts. 

Wind and Target Motion 

Air-to-ground fire-control systems in use at the 
end of the war made provision neither for motion of 
the target on the ground, nor for wind. It was found 
in the 75-mm cannon tests that the lateral component 
of the wind was easily corrected for by the pilot, who 
chose an aiming point to the right or left of the true 
target. The component of the wind or target motion 
in the direction of the flight path was harder for the 
pilot to judge and nullify. Further developments 
should take care of this problem if high precision at 
long ranges is desired. 

The Effect of Skid in Rocket Fire 

Tests on the use of radar range systems in air-to- 
ground rocket fire at long range had been commenced 
but a factor which is of minor importance with can- 
non fire here became a serious problem. This is the 
problem created by the skidding or side-slipping of 
the airplane ; the rocket launcher points in a direction 
different from that in which the airplane is moving. 
The seriousness of this problem arises from the fact 
that a fin-stabilized rocket, unlike a shell, leaves the 
launcher practically in the direction in which the air- 
plane is moving rather than in the direction in which 
the launcher is pointed (that is, it heads into the 
relative wind). In actual tests it is observed that 
occasional rockets go very far to the side of the 
target. 

There are two conventional means by which skid 
has been estimated. One is the feeling in the seat of 
the pilot’s pants, the other is a bubble, free to move 
in a curved tube of liquid. It is known, however, that 
under some conditions a skid of fairly sizable amount 
(2 degrees) will escape detection by both of these 
means. 87, 107 

Some devices were under development at the end 
of the war which would detect skid, and others which 
would correct it. The former were much more nearly 
perfected than the latter. The “Barber Pole” of the 
Specialties Company is an example. A yaw head 
with two openings is mounted on a wing of the air- 
plane. Rubber tubes lead from these to a cylinder 
with helical stripes, mounted horizontally within the 
pilot’s field of vision. When a pressure differential 



274 


ASSESSMENT PROBLEMS 


exists between the openings, the cylinder is caused 
to rotate in the direction of skid, attracting the 
pilot’s attention. While this does not give the amount 
of skid, it does allow the pilot to correct for it as soon 
as it begins. A sensitivity of 2 mils is attainable. 
However, this would be too sensitive for practical 
use, and damping would be required. 

22.4 GENERAL PHILOSOPHY OF 
ASSESSMENT 

The concept of a test as a single, simple occurrence 
to answer the question, “Do we want it?” has long 
since been outmoded. The modern view regards test- 
ing as a process that goes on continually, from the 
first experimental model, through the development 
stages, the small scale application stage and does not 
stop even when large scale application is reached. 
The success of characteristic American peacetime 
engineering accomplishments is partly owed to this 
process. The surprising backwardness of German 
radar development is credited to the fact that the 
scientists who invented the equipment had no contact 
with it after it left their hands, a situation which 
was apparently true in many cases with the manu- 
facturers as well. The desire for military security was 
allowed to stifle the free exchange of ideas secured in 
experimental use, by which the designers and manu- 
facturers could have made the rapid advances which 
characterized British and American military tech- 
nology. 

22.4.1 Need for Experimental and 
Engineering Testing 

In spite of the admirable way in which practical 
experience was frequently made available for the 
scientific workers who were engaged in the develop- 
ment of new devices, fire control was not improved 
as it might have been. The philosophy of the accept- 
ance test, pure and simple, delayed matters. A fire- 
control system would be developed in the laboratory, 
tested out with the limited facilities available to the 
development agency, which usually did not include 
ranges where firing was possible, and then sent to the 
testing agency where, for the first time, firing tests 
were conducted. The natural failings of a new system, 
and the lack of training of the testing personnel would 
result in an unfavorable test report. This might hold 
up matters for a number of months, or, if someone in 
high authority had faith in the system, it would be 
placed in production in spite of the adverse opinion. 


Toward the end of the war a more efficient attitude 
was adopted, for instance at Eglin Field, where a 
distinction was made between engineering tests and 
final acceptance tests, and some systems were given 
the experimental or engineering type of test while in 
the small-scale manufacturing stage, then modified 
in such manner as the test results showed necessary. 
The value of direct experience to the scientist or engi- 
neer working on a system is great enough to suggest 
that a few such persons should be trained to fly 
fighter aircraft. There seems to be a disagreement 
among authorities as to whether this is feasible or 
not. 

22.4.2 Limitations of Testing 

Methods 

However thorough the engineering and acceptance 
testing may be, it cannot give all of the information 
needed to make an intelligent decision about the 
usefulness of the system in actual combat. Operations 
to be performed by human operators have been 
shown to be much less accurate under combat con- 
ditions than under test conditions. Test pilots be- 
come familiar with the terrain of their own ranges; 
they may be affected by prejudices for or against the 
equipment; they will probably fly at lower speeds 
than they would under combat conditions. Errors 
that compensate one another under test conditions 
may not compensate in combat, and vice versa. 
Maintenance crews may be less careful about fire- 
control equipment under test than when they know 
that their lives may depend upon its being in the best 
possible condition. 

22.4.3 Combat Trials 

The above considerations lead to the idea that 
testing under combat conditions would be desirable. 
Whether this is possible depends upon circumstances. 

Contrast between Offensive and Defensive 
Tests 

It is clear that any system to be used in offensive 
combat can be given a test at almost any desired 
time, unless the enemy has collapsed. On the other 
hand, it may be unwise to introduce new offensive 
equipment in the development stage because its pos- 
sible loss over enemy territory would compromise 
the development. Defensive equipment cannot al- 
ways be subjected to the same ready testing. For 
example the AN/APG-5 system with the K-15 sight 


% RESTRICTED J' 


GENERAL PHILOSOPHY OF ASSESSMENT 


275 


was installed in a few bomber turrets in the Medi- 
terranean theater, and flown on combat missions, but 
it never received any real trial as the German fighters 
did not attack. 

Use of Small, Specially Equipped Groups 
Ideally a small combat test group would be or- 
ganized as soon as practicable, given special training 
in the operation and maintenance of the system, and 


sent to a theater of operations. Trained observers 
would accompany the group with facilities for 
photography and other means of securing test data, 
and the results of such observations would be utilized 
by the development agency as rapidly as possible in 
the further improvement of the equipment. Needless 
to say, a decision on the overall acceptability of the 
system could not await such a test, for the time lag 
would be too great. 


RL 


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PART V 


AIRBORNE MOVING 

INDICATORS 


TARGET 


RESTRICTED # 




Chapter 23 

BASIC PRINCIPLES OF AMTI 


23.1 INTRODUCTION 

One of the most important radar implements of a 
modern tactical air command is an airborne device 
capable of detecting moving vehicles of all descrip- 
tions at night or during periods of poor visibility. For 
example, the lack of such a device cost the Allied 
forces tremendous losses in the Battle of the Bulge, 
because they were not fully prepared for the vast 
forces that crept in against them under cover of the 
ten-day period of foggy weather during which the 
normal type of aerial reconnaissance was impossible. 
The usual type of airborne search radar does not pro- 
vide a means for distinguishing moving vehicles from 
ordinary ground clutter because it works on the basic 
principle of target contrast. 

The pulse doppler phenomenon, however, does 
provide a means for distinguishing between moving 
vehicles and ground clutter. The basic principles of 
detection depend upon the fact that the relative mo- 
tion, with respect to the radar transmitter in the air- 
craft, is different for fixed ground targets than for 
moving targets. 2-4 In particular, an airplane flying 
over the land receives echoes from the the ground 
with frequencies shifted slightly from the transmitter 
frequency because of the relative motion of the 
ground and the aircraft. The doppler frequency shift 
is approximately 30 c at X band for a radial velocity 
of 1 mph. Thus, for an airplane velocity of 200 mph 
the shift is approximately 6,000 c for echoes fore and 
aft and 0 for echoes abeam. If the vehicle on the 
ground has a radial component of velocity toward 
the airplane, there will be an additional shift of fre- 
quency corresponding to the total radial components 
of velocity of airplane and vehicle. For example, if 
the vehicle has a radial velocity toward the aircraft 
of 20 mph and lies ahead of the aircraft, the total 
shift of frequency will be approximately 6,600 c. 
Normally the frequency shift is not detected by the 
radar in the aircraft because it is negligible compared 
with the bandwidth of the receiver. However, if the 
returning signal is allowed to beat with an oscillator 
whose frequency is the same as that of the trans- 
mitter, this beat-note can be detected and its fre- 
quency will correspond to the velocity of the vehicle 
with respect to the aircraft. If the frequency of the 
oscillator is equal to that of the transmitter plus the 
doppler frequency shift corresponding to the motion 


of the aircraft over the ground, then the beat-note 
frequency will correspond to the velocity of the 
vehicle with respect to the ground. 

23.2 THEORY 

23 . 2.1 Doppler Principle for Moving 
Vehicle Detection 

The doppler frequency shift 2 3 at the transmitter- 
receiver caused by the relative radial velocity, V, of 
the target is approximately 



/ = 29.8 cycles/sec/mph at A = 3 cm. (1) 

The shift may be detected by beating the echo to- 
gether with an oscillator whose frequency is the same, 
or nearly the same, as the frequency of the trans- 
mitter, and in other related ways discussed in what 
follows. 

Continuous- Wave Transmission Method 

The simplest application of the doppler principle 
to a detection device for moving vehicles is that in 
which a continuous-wave [CW] transmitter with its 
antenna is set up adjacent to the receiver and its 
antenna. 4 The echo returning from the target, as 
well as some direct energy from the transmitter, 
enters the receiver and the ensuing beat-note indi- 
cates target motion. Although this is a very sensitive 
method for detecting moving targets, it does not give 
their ranges nor does it discriminate among them 
except in azimuth (then only by virtue of the an- 
tenna pattern) . It is not readily applicable as an air- 
borne moving vehicle detector because of the diffi- 
culty of separating the indications coming from the 
ground from those coming from the moving vehicle. 

Noncoherent Pulsed-Transmission Doppler 
Method 

In order to obtain the range of the target in addi- 
tion to its azimuth, it is convenient to resort to a 
pulsed system such as an ordinary radar. 25 ’ 2J When 
the airborne radar is used over land, the frequency 
of the echoes returning from the random fixed scat- 
tered on the ground is shifted from the transmitter 
frequency according to the relation given above; 


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279 


280 


BASIC PRINCIPLES OF AMTI 



Figure 1 . Photograph of the cathode-ray tube in the Firefly (AN/APS-27) moving vehicle detector, showing a line 
of moving vehicles on the Newburyport Turnpike, and a few isolated moving vehicles. The range circle is at 5 miles 
radius. 


and, as mentioned in the introduction, the shift can- 
not be detected by the ordinary radar receiver. If a 
moving vehicle lies within the field of view of the 
radar the situation is different; the returning energy 
is the vector sum of the energy of the scatterers near 
the vehicle and the energy from the vehicle itself; as 
the vehicle moves toward or away from the aircraft, 
the phase of its echoes changes with respect to the 
phase of the echoes from the background by 180 de- 
grees for every quarter wavelength or 0.3-in. ra- 
dial motion of the vehicle (at X band) and the echo 
energy from the vehicle alternately adds and sub- 
tracts from the average energy of the region. The 
periodic amplitude modulation or flutter of echoes 


so often seen on the A scope is evidence of the motion 
of the target with respect to its background (slow 
fluctuations of a few cycles per second are usu- 
ally due to frequency shift of the transmitter or 
inhomogeneities in the refractive index of the air). 
The frequency of the beat-note modulation is given 
by the relation / = 2E/A, where V is now the radial 
component of the velocity of the vehicle with respect 
to the ground. 

The term noncoherent doppler detection is given to 
the above phenomenon because no coherence or es- 
tablished phase relationship is required between the 
oscillations from the transmitter and from the echo. 
The phenomenon has the property that the motion 


RESTIUCTELL 




THEORY 


281 


of the aircraft relative to the ground does not result 
in modulation of background echoes except as a 
secondary effect ; only targets moving with respect to 
the ground show the modulation. Consequently, it 
is possible to detect vehicles at all azimuth angles 
provided that they have sufficient radial velocity 
with respect to the instantaneous position of the 
aircraft. An illustration of noncoherent doppler mov- 
ing vehicle detection is shown in Figure 1. 

In addition to the modulation frequencies caused 
by the presence of moving vehicles, there will also 
be some modulation frequencies because of the finite 
width of the beam. These modulation frequencies 
will be present in the return from random fixed scat- 
tered because of the differential radial velocities. 
The case for scatterers located at a given range and 
at the half-power points of the beam is illustrated 
in Figure 2. 



Figure 2. Noise modulation caused by finite beam- 
width. The modulation is minimum for <f> = 0 or 
180 degrees and maximum at 0 = 90 or 270 degrees. 


The radial component of velocity of a scatterer 
at point A is 

Vi = V cos (^<f> - ^ , 

at point B 

V 2 = V cos ^</> + , 

and 

A 

Vi — V 2 = 2V sin <f> sin - . (2) 


The corresponding modulation frequency is 

, 4V . .A 
f = — Sin <f> sin - (3) 


which is maximum when </> = 90 or 270 degrees (i.e. 
abeam). In the case of a 3-cm radar with 29-inch 
parabola, A = 3° = 3^0 radian, and for V = 200 mph, 
/ max = 300 c (equivalent to 10 mph). In practice, 
beamwidth modulation results in a spectrum of fre- 
quencies from 0 up to several hundred cycles per 
second and appears as noise on the A scope, render- 
ing it somewhat more difficult to detect moving 
vehicles abeam than fore or aft. A plan position indi- 
cator [PPI] photograph showing “ wings” due to 
beamwidth noise is shown in Figure 3. 

In taking full account of beamwidth effects it is 
also necessary to consider differential radial veloci- 
ties existing in range between the fixed scatterers at 
a given azimuth angle. An expression somewhat 
similar to the above may be derived for beamwidth 
modulation as a function of tilt-angle of the beam, 
indicating that at close ranges and steep angles the 
noise frequency spectrum is of the same order of 
magnitude as that noted above. 

In the case of the pulsed radar system, the doppler 
frequency must be different from the pulse recurrence 
frequency [PRF] in order to be detected. They will 
be equal when the target moves exactly one-half a 
wavelength toward the aircraft between pulses, in 
which case there will be no change in relative phase 
between target and background echoes and no energy 
modulation will occur. Similarly, the target will be 
undetectable when the modulation frequency is an 
integral multiple of the PRF. In general, blind speed 
regions are given by 


F blind 


n • PRF • X 
2 


= n • 67 mph 


(4) 


where n = 1, 2, 3, • • • at X = 3 cm, PRF 2,000. 

Thus for a system of the above characteristics, 
blind speeds will occur in the regions of 67, 134, 201 
mph (see Figure 4). Although theoretically it is im- 
possible to detect vehicles whose velocities lie within 
the blind speed regions, it is still true that fluctu- 
ations in the speed of the vehicle, in the disposition 
of the random scatterers in the background, and 
mutual interference between various reflecting sur- 
faces of the vehicle itself will, for all practical pur- 
poses, eliminate the blind speed regions. 


282 


BASIC PRINCIPLES OF AMTI 



Figure 3. Firefly scope. One-mile range marks. Moving vehicles on the Worcester Turnpike. Noncancellation 
“wings” caused by beamwidth modulation noise show that noncancellation effects are greatest abeam. 


Coherent Pulsed-Transmission Doppler 
Method 

The fundamental difference between the nonco- 
herent and coherent methods 4 * 39 lies in the location of 
the source of oscillations with which the moving tar- 
get echo is mixed to produce the beat-note. 2 3 In the 
coherent pulsed doppler system the source is a CW 
coherent oscillator QCOHO] in the radar itself. Use is 
made of the fact that each pulse which returns from 
the echo bears a definite phase relationship with re- 
spect to the transmitted pulse. The COHO provides 
the means for measuring this phase relationship. This 
oscillator can either drive the transmitter which is 


then a power amplifier or be driven by, or in phase 
with, the transmitter. At microwave frequencies the 
most successful coherent pulsed doppler system to 
date has used a CW coherent oscillator which is 
driven by the magnetron in the following way. The 
r-f transmitter pulse, at 2,970 me, is mixed with 3,000 
me r-f energy from a very stable local oscillator to 
form a pulsed beat-note at the intermediate fre- 
quency (30 me). This pulse is then used to start up 
as well as to control the starting phase of a 30-mc 
oscillator which feeds into an i-f mixing channel in 
the receiver. The starting phase of the 30-mc oscil- 
lator (the COHO) will depend upon the relative 



THEORY 


283 


X *3 cm 
PRF *2000 



Figure 4. Receiver output versus radial velocity of 
target. The “blind speed regions’’ are at 67, 134, 
201, etc., mph. 


phases of the transmitter and the stable local oscillator 
[STALO]; if they are initially in phase, the starting 
phase of the COHO will be at maximum amplitude; 
if they are 180 degrees out of phase, it will be 0, etc. 
In any case, the coherent oscillator will bear a definite 
phase relationship to the combined phases of trans- 
mitter and stable local oscillator at the instant of 
initiation of the transmitter pulse. 

The return echo at 2,970 me beats with the same 
stable local oscillator to produce a 30-mc beat-note 
(i-f) whose starting phase depends upon the com- 
bined phases of the echo and the stable local oscil- 
lator. From pulse to pulse, this starting phase will 
vary from 0 to 360 degrees, depending upon the phase 
of the stable local oscillator. At the same time, the 
starting phase of the coherent oscillator will have 
passed through identical phase shifts, with the net 
effect that the phases between the coherent oscillator and 
echo beat-note bear a fixed relationship to one another 
from pulse to pulse. Their phases relative to the 
transmitter pulse may vary from 0 to 360 degrees, but 
their phase difference will not vary from pulse to 
pulse. When the i-f and the 30-mc coherent oscilla- 
tions are mixed in the detector circuit of the receiver, 
they will add or subtract and the signals will appear 
on the A scope as either up or down, but will not 
vary in fixed amplitude from pulse to pulse. 

If the vehicle producing the echo is in motion, the 
phase relationship will not remain fixed from pulse 
to pulse, and the signals on the A scope will show 
amplitude modulation of frequency / = 2F/X, 
spreading out, both up and down. This fact dis- 
tinguishes the moving from the fixed targets and per- 
mits their echoes to be separated by methods to be 
described in Section 23.2.2. 


In an airborne coherent pulsed doppler system all 
fixed scatterers on the ground except those abeam 
show motion and consequent amplitude modulation 
of frequency 23 28 

2V 

fe = ~~ cos 6 , (5) 

A 

where 6 is the azimuth angle the beam makes with the 
direction of flight and V g is the ground speed of the 
aircraft. All the echo pulses in the entire “string” of 
pulses are identically shifted in phase with respect 
to the coherent oscillator from pulse to pulse be- 
cause of the motion. The phase shift corresponds to 
f e . Consequently, if the phase of the coherent oscil- 
lator is shifted a similar amount from pulse to pulse, 
the phases between coherent oscillator and the in- 
dividual beat-notes will again bear a fixed relation- 
ship to one another, and the deflections on the A 
scope will again stay constant, up or down, except 
for moving targets. The coherent oscillator phase 
shift is accomplished by adding to the coherent 
oscillator frequency an audio frequency correspond- 
ing to f e so as to produce a new coherent oscillator 
frequency of 30 me + f e . Thus the effective starting 
phase of the coherent oscillator will be the sum of 
the three phases of transmitter, stable local oscillator, 
and audio-frequency phase shifter. The phase shifter 
computing box is tied in with the azimuth gearing of 
the radar scanner so as to produce the proper vari- 
ation of f e with azimuth angle. A manual adjustment 
is provided for setting f e to the appropriate value for 
reducing the general clutter modulation to a mini- 
mum. 

The coherent pulsed doppler system has the ad- 
vantage over the noncoherent system in that it is 
possible to detect moving vehicles, aircraft, and ships 
in the absence as well as in the presence of ground 
clutter. Furthermore, as explained in the next section, 
the phase detection device is inherently superior to 
an amplitude detection one because phase fluctu- 
ations in extended ground clutter, caused either by 
scanning or by wind, are independent of the size of 
the clutter. 42 Therefore, the clutter produces a uni- 
form residue which can be set up equal to noise. 
The sensitivity to a target moving through the 
clutter is independent of the phase of the clutter. 

23.2.2 Cancellation and Presentation 

The object of the noncoherent and coherent systems 
outlined above is to produce video (or i-f) echo signals 
which are amplitude-modulated from pulse to pulse 


284 


BASIC PRINCIPLES OF AMTI 


for moving targets and fixed in amplitude for sta- 
tionary ones. The simplest form of presentation of 
this information is the A scope on which the sta- 
tionary target signals stand up or down while the 
moving ones have the appearance of a butterfly. 

By gating the video a modulated echo can be iso- 
lated from the others in time. The modulation can 
be detected in a pair of headphones. By passing the 
gated echo through suitable pulse-stretching and 
clamping circuits, the recurrence frequency can be 
almost completely eliminated leaving only the audio 
modulation signal. This is the principle of the Butter- 
fly system (AN/APS-26) 29 to be described later. In 
the Firefly system (AN/APS-27) 25 and airborne mov- 
ing target indicator [AMTI] system for Cadillac, 28 the 
modulation on the echoes is detected by sending the 
radar video signals down a mercury delay line or 
into a storage tube which stores them for a period of 
one recurrence cycle and compares them in amplitude 
with the string of echoes from the next pulse. The 
result of this operation is to cancel out all those 
echoes which are relatively unchanged in amplitude 
from pulse to pulse and let through those that differ 
in amplitude from pulse to pulse. These residual sig- 
nals are then displayed on the plan position indica- 
tor of the radar in the normal manner. 

A word of warning is introduced here for those 
contemplating using a noncoherent pulsed doppler 
system for bombing purposes where accurate target 
range is required. The range of the leading edge of the 
cancelled video from a moving target echo depends 
not only upon the range of the target but also upon 
the range of the dominating clutter echoes adjacent 
to the target. As a result, some uncertainty and fluc- 
tuation will be introduced into the range measure- 
ments. The effect bears investigation in problems 
where range measurements more accurate than one 
pulse width (400 ft) are required. 

23.2.3 Limits of Detectability 

The output amplitude of the cancelled video is a 
function of the velocity of the vehicle and the receiver 
characteristics. Theoretically, the amplitude drops 
to zero in the blind speed regions as shown in 
Figure 4. 

The ability of the system to discriminate between 
moving targets and background clutter is a complex 
function of the properties of the clutter itself (mag- 
nitude and phase), characteristics of the receiver, 
cancellation unit, presentation, and precision of ad- 
justment of the temporal and amplitude cancellation. 


Although the question is debatable, at present it 
appears that phase detection methods will be su- 
perior to amplitude detection (noncoherent doppler) 
methods 2> 3> 31 for separating moving target echoes 
from ground clutter background, which is subject to 
large amplitude and phase fluctuations. On the other 
hand, phase detection has the disadvantage that it 
requires much more instrumentation than noncoher- 
ent amplitude detection. The system will be more 
difficult to operate and there will be more manual 
controls for the operator to adjust. Furthermore, at 
close ranges the ground clutter velocity, which must 
be cancelled out, changes appreciably with range and 
the velocity computer box becomes unduly compli- 
cated. 

Theory indicates that the sensitivity of the phase 
detection scheme utilizing pulse-wise comparison is 
such that owing to spurious changes caused by noise, 
equipment instability, ground-clutter fading, and 
scanning, moving targets which are more than about 
25 db weaker 3 than the underlying ground echoes 
cannot be detected by methods operable at accept- 
able scanning speeds. Experimental verification of 
this figure has not yet been obtained. 

Minimum detectable speeds and minimum sepa- 
ration of vehicles for resolvable signals have been 
determined for only one system to date, namely 
Butterfly. Vehicles traveling at 4 mph have given 
satisfactory indications. Targets 250 ft apart were 
separable as individual signals on the A scope. In 
ground tests a single man walking at about 2^ or 
3 mph was readily detected and the rhythmic fluctu- 
ation of the tone corresponding to each individual 
footstep was easily discernible. * 

23.3 SYSTEM DESIGN CONSIDERATIONS 

23.3.1 The Radar System 

Any imperfection in the radar itself that shows up 
as modulation of the video signals will reduce the 
overall sensitivity of detection of weak signals. In 
the radar, hum due to pickup can be eliminated by 
operating certain of the tube filaments (especially 
those in the receiver) on the aircraft’s d-c supply, 
and hum due to incomplete filtering of the power 
supplies can be eliminated by additional filters. 
In high-powered radars the additional filtering im- 
poses a serious space and weight burden. An alterna- 
tive method whereby the effects of the hum on the 
output can be eliminated without actually elimi- 


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SYSTEM DESIGN CONSIDERATIONS 


285 


nating the hum itself is to synchronize the recurrence 
frequency of the radar to the power supply frequency 
(400 or 800 c) or to twice the power frequency (800 or 
1,600 c) so that each transmitted pulse occurs at the 
same point on the a-c cycle. This scheme requires 
either that the storage device be independent of the 
recurrence interval, for example, a storage tube of 
some type, or that the a-c frequency be regulated 
within narrow limits. The latter has not yet been 
achieved in practice. 

Triggering and circuit jitter is another serious 
source of modulation. Spark gap modulators cannot 
be used. The transmitter jitter must be kept less 
than 0.02 ix sec for satisfactory cancellation. 

In scanning systems the antenna is designed to 
provide a fan or cosecant-squared beam pattern in 
the vertical plane so as to produce adequate ground 
illumination, and in the horizontal plane as narrow 
a pattern as possible in order to maintain a good 
signal-to-clutter ratio. 14 The sensitivity of the system 
is a function of the scanning rate, beamwidth, type of 
storage device, and other variables. For example, in 
pulse-to-pulse comparison, a high scanning rate to- 
gether with a narrow beam will result in loss of sensi- 
tivity because of the shift in beam between pulses, 
with resulting partial noncancellation of fixed echoes. 
Frame-to-frame comparison devices, 6 * 10 such as mo- 
saic storage tubes, permit a high scanning rate, but 
the excessive time interval between looks renders 
them impractical for airborne systems. 

The selections of the transmitter wavelength X 
and its PRF determine to a large extent the 
operational characteristics of the system. The PRF 
establishes the maximum possible range. By reference 
to Figure 4 it may be seen that X and the PRF es- 
tablish the blind speed regions and the minimum 
detectable vehicle speed. The X and antenna diameter 
determine the beamwidth and hence the beamwidth 
modulation. For X band, a reasonable compromise 
between the positions of the blind speed regions, 
beamwidth modulation, and minimum detectable 
speed (about 4 mph) was arrived at by making the 
PRF 1,600 c. A minimum detectable speed of less 
than 4 mph (doppler frequency, 120 c at X band) is 
probably undesirable for an airborne system because 
of beamwidth, modulation, clutter fluctuations, and 
a-c hum. 

A receiver of considerable dynamic range (40 to 
60 db) is necessary if good discrimination between 
weak signals and clutter is to be attained over a wide 
range of clutter signal strength. 26 * 27 Modulation ob- 


viously cannot be detected if the ground clutter sig- 
nals saturate the receiver intermediate frequency. 
Theoretically, the best type of receiver appears to be 
one having a linear characteristic at low input levels 
and a logarithmic characteristic thereafter. This 
subject is still open to question. 8 * 26 

23.3.2 The Cancellation Unit 

Although considerable work has been done on de- 
lay-line and storage tube types of cancellation units, 
much more development is required before either 
type can be properly evaluated. It is possible to ex- 
clude from this discussion the mosaic-screen frame- 
wise cancellation device because with present scan- 
ning rates, the losses in frame-by-frame comparison 
are prohibitive. 10 Pulse- wise storage devices are di- 
visible into two classes: those that require precisely 
controlled recurrence frequency (delay-line type) 
and those that store the string of video pulses for an 
indefinite period (storage tubes). 

Delay-Line Type 

The design of the delay line 9 * 12 * 15> 16 * 18 * 19 - 2l * 22 > 25 « 
27 , 33, 34 j s intimately bound up with the problem of 
accurately controlling the recurrence interval of the 
pulses so that the video signals emerging from the 
cancellation unit are superimposed exactly on the 
video signals from the succeeding pulse. Two general 
schemes have been employed in airborne equipment 
for automatically or semiautomatically maintaining 
the proper recurrence rate. 

One scheme 28 employs a separate delay line, 
slightly shorter than the video delay line, as a time 
element in a regenerative trigger circuit. The total 
time delay in sending a triggering pulse down the short 
line, through the amplifiers and a blocking oscillator 
to initiate the succeeding triggering pulse, is made 
exactly equal to the time delay interposed by the 
video delay line. The modulator is triggered by the 
timing circuit; time delays in the modulator trigger 
are unimportant provided they are constant because 
they result in delaying successive strings of video 
pulses by identical amounts. Another form 25 of this 
same scheme is a 3-crystal delay line in which a 
separate pick-off crystal inserted slightly ahead (0.1 
Aisec) of the video pick-off crystal provides the 
proper time delay for the timing circuit. 

The other scheme involves using a simple 2-crystal 
delay line 25 for timing as well as for the video delay. 
The triggering pulses in the timing loop are generated 
by a free-running phase shift audio oscillator whose 


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BASIC PRINCIPLES OF AMTI 


frequency can be controlled to within 0.004 per cent 
by the voltage developed in a tracking and coinci- 
dence circuit. This arrangement can be adjusted to 
provide completely automatic temporal cancellation 
thereby relieving the radar operator of one of his 
many duties. 

Storage Tube Type 

A deflection-modulated 5-inch cathode-ray tube 25 
operated in a manner similar to an ordinary A scope 
has been employed successfully as a storage device. 
Video signals from the radar are applied to the de- 
flection plates of the tube in the usual manner. The 
signals are taken from a capacity pickup screen at- 
tached to the external face of the tube as shown in 
Figure 5. This arrangement has the surprising prop- 
erty that only differential signals due to amplitude 
modulation of the radar signals are picked up, ampli- 
fied, and transmitted on to the PPL At present, the 
phenomenon is not completely understood. The fact 



Figure 5. Cancellation by an A-scope storage tube. 


is that as long as the beam accurately retraces its 
path, no sudden change in charge appears on the 
pickup screen, but when the beam deviates from its 
previous path, corresponding to a fluctuation in 
amplitude of the signal, then a pulse is observed at 
that time. The great advantage of the storage tube is 
that it permits the PRF to be synchronized with the 
power supply. It would also permit the PRF to be 
varied or staggered so that blind speed regions could 
be eliminated. 

Additional information on subjects closely related 
to airborne moving vehicle detectors is contained in 
references listed in the bibliography for Part V. 


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Chapter 24 

AIRBORNE MOVING TARGET INDICATOR SYSTEMS 


24.1 BUTTERFLY (AN/APS-26) (AUDIO 

PRESENTATION) 

24.1.1 General Description 

From the technical standpoint, the simplest air- 
borne moving vehicle detector is the Butterfly system 
(AN/APS-26) 29 which detects by the noncoherent 
pulsed doppler method and presents the information 
as an audio tone in the radar operator’s headphones. 
In operation, the radar beam is merely pointed in the 
direction of flight of the aircraft, and the range gate 
(about 500 ft wide) is set to a point approximately 2 
to 3 miles ahead of the aircraft. Only those moving 
vehicles in the narrow strip approximately 3,000 ft 
wide and 500 ft in range produce beat-tones in the 
headphones. As the aircraft flies along its course, 
vehicles lying within the 3,000-ft swath are inter- 
cepted, one by one, and they produce in the head- 
phones short musical tones from 1 to 2 sec duration. 
The pitch of the tone indicates the radial component 
of the velocity of the vehicle relative to the ground, 
and an experienced operator can make inferences 
about the type of vehicle by the character of the 
note; tanks, trucks, and similar vehicles will produce 
strong low-pitched signals; jeeps and faster moving 
vehicles, higher-pitched ones. Some typical indica- 
tions are shown in Figure 1. 

From the tactical standpoint, Butterfly is some- 
what limited in usefulness. An aircraft carrying the 
equipment must be under a close control system, for 
example, the SCR-584/M with MC-627 plotting 
equipment, so that it can be directed precisely 
enough over roadways, railroads, or waterways to 
assure that they lie within the 3,000-ft strip. At first 
sight, this limitation may appear to be a staggering 
disadvantage, but when the entire tactical picture 
is visualized, it will be seen that close control of the 
aircraft is desirable in any case. In an operational 
mission the operator radios back to the control sta- 
tion when he “hears” the vehicles, and the plotters 
at the station then mark the location of the vehicles 
on the map. Knowing the position of the vehicles to 
within a few hundred yards, their approximate num- 
ber (convoy or individual vehicles) and roughly their 
type, the tactical planning section is in a good posi- 
tion to act immediately and send ground-controlled 
bombers or strafers to the region, or to plan a future 


attack upon the depot, dump, marshalling yards, or 
other concentrations as indicated by the influx or 
efflux of the traffic. 

The tests of Butterfly under close control of an 
SCR-584/M and plotting board at the AAF testing 
ground, Eglin Field, Florida, showed that even with 
a preliminary low-power model of the Butterfly hav- 
ing a narrow beam and limited range, close control 
down roadways with adequate precision to keep the 
beam on the road was tactically feasible. 

24.1.2 Technical Description 

The Butterfly system is essentially a very simple 
radar consisting of the components shown in Figure 
2. A brief description of the components of the Radi- 
ation Laboratory prototype follows. 

Radar 

The transmitter-receiver is an AN /APS-15A modu- 
lator, modified by the addition of an extra resistance- 
capacity [RC] filter across the local oscillator voltage 
supply. 

The antenna consists of a standard AN/APS-15A 
base and X band plumbing and an AN/APA-46 
servo control unit for precise positioning in azimuth. 
The standard antenna feed is supplanted by a new 
one which gives a beamwidth of 9 degrees in azimuth 
and 3 degrees in elevation. The parabola is blocked 
off with “harp” (absorbing) material along the verti- 
cal side in order to attain this pattern. Tests show 
that the 9-degree beam introduces no serious beam- 
width modulation noise, and simplifies the problem 
of keeping the beam on the roadway during turns 
and banks of the aircraft. 

The receiver is an AN/ APS- 10 model modified by 
the addition of automatic gain control derived from 
the gated output signal. Its function is to keep the 
signal level below saturation so as to preserve pulse- 
to-pulse amplitude modulation. 

The power supply is an 800-1-C, 115-v, 800-c con- 
verter. It establishes the pulse recurrence frequency 
[PRF] of the system at 1,600 c. As explained in 
Chapter 23, synchronizing the PRF with the power- 
supply frequency in this manner eliminates the effect 
of insufficient filtering, a-c pickup, and other factors 
by firing the modulator each time at exactly the same 
point on the a-c wave. 


287 


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i 


288 


AIRBORNE MOVING TARGET INDICATOR SYSTEMS 




E WOMAN WALKING F MAN AND CAR (SMOOTHED OUTPUT) 


Figure 1. Characteristic signals from moving targets, as detected by Butterfly (AN/APS-27), measured at the audio 
amplifier output. 


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BUTTERFLY (AN/APS-26/ (AUDIO PRESENTATION) 


289 



Figure 2. Butterfly (AN/APS-26) moving vehicle de- 
tector. Simplified schematic block diagram. 


Synchronizing and Triggering 

A simplified block diagram of the synchronizer and 
trigger unit is shown in Figure 3. Video from the 
AN/ APS- 10 receiver strip passes through a l-/isec 
gate whose position relative to the transmitted pulse 
is varied by a potentiometer in the variable delay 
circuit. Usually the gate is set to a point correspond- 
ing to 2 or 3 miles ahead of the aircraft. After passing 
through the gate, the video pulses from the echo are 
stretched out by a clamping circuit so as to produce 
the envelope of the peaks of the pulses. This audio 
wave is amplified and passes into the headphones. 

Presentation 

In addition to the Butterfly signals, information 
from the aircraft intercommunication system passes 
into the radar operator’s headphones so that he can 
hear the pilot or the radio simultaneously for liaison 
or warning purposes. The operator also has an A 
scope for visual presentation of the ungated video 
signals. The scope has a delayed fast sweep (6,000 
yd) and a slow sweep (24,000 yd) . The slow sweep is 
used for positioning the antenna along the direction 


of flight by means of the Nosmo (AN/APA-46) null- 
doppler technique, and for adjusting the antenna 
tilt so that the center of the beam strikes the ground 
at the position of the range gate. The fast sweep is 
used in conjunction with the headphones to aid the 
detection of the moving targets which are easily dis- 
tinguished from ground clutter by their “ butterfly” 
appearance. 

Operation 

Aside from the usual power switches, tuning and 
gain controls, the radar operator has at his disposal 
antenna positioning controls, a range-gate position- 
ing control, an audio gain control and a switch for 
selecting manual or automatic receiver gain control. 
The principal adjustments during flight are antenna 
positioning in tilt (which rarely exceeds 30 degrees 
because beamwidth noise then becomes objection- 
able), antenna positioning in azimuth to allow for 
drift angle, and range-gate position. Although one 
might expect the radar operator to be satisfied to 
leave the range-gate position control set at a specified 
range for a given altitude, experience has shown that 
this is not the case. The variable range-gate control 
makes it possible for the operator to quickly scan 
the entire range, say from 1 to 10 miles ahead, for 
moving targets, and once a moving target is located 
to keep the target in the gate for a comparatively long 
period in order to learn as much as possible about it 
from the pitch and quality of its “musical” note. 

In addition to the operating controls, the set is 
equipped with a built-in modulated echo-box testing 
device with which it is possible to make a routine 
overall performance check on the system, on the 
ground or in flight, with a modulated r-f signal 



Figure 3. 


Butterfly synchronizer and detector. 

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290 


AIRBORNE MOVING TARGET INDICATOR SYSTEMS 


simulating that from a moving vehicle. Such a test 
is extremely important. A detuned or otherwise in- 
efficiently operated radar set is worse than no radar 
at all in this new type of radar reconnaissance mis- 
sion, for although it is not customary to act on 
negative information still nonindication of vehicles 
might be interpreted by the military as meaning “no 
vehicles at all.” This could lead to a false sense of 
security concerning the military situation in the 
region under surveillance. 

24.1.3 Performance 

Potentialities and Limitations 

Some general performance figures for the Radiation 
Laboratory prototype are: 

Maximum range: 10 miles (at least) on individual 
vehicles. 

Minimum vehicle speed: 4 mph. 

Minimum separation of vehicles: 250 ft for re- 
solvable signals. 

Maximum operating altitude: Over 18,000 ft. 

Width of strip covered by beam: About 9 degrees 
(3,000 ft at 3 miles) for medium strength signals, 
much wider for strong ones. 

Ship detection: Possible in sea clutter or along 
shore. Ordinary radar can be used in open sea. 

The chief limitations of Butterfly are that it is not 
all-around-looking, and that close control of the air- 
craft is required. On the other hand, the system has 
high sensitivity and it gives an indication of the 
speed (hence, to some extent, the type) of the 
vehicle, which is something that no other system 
provides. 

Ground Applications 

When the system is stationary on the ground there 
is no beamwidth noise modulation and it is conse- 
quently possible to detect weak signals from very 
slowly moving objects as well as vehicles, low-flying 
aircraft, etc. A single man walking at 3 mph is 
readily detectable out to about 2,000 yd range. Each 
footstep is distinguishable by the pitch modulation 
of the ensuing note. Vehicles are detectable 2 to 3 
miles away, the only requirement being that line of 
sight be maintained. It is believed that the ground 
application of the Butterfly device will prove to be 
important in patrol activities and other battle-line 
problems. 


Characteristic Indications 

The operator soon learns to tell something about 
the moving target from the characteristic quality of 
the note produced. The oscillograms in Figure 1 
illustrate to some extent the influence of various 
factors upon quality. 

24.2 FIREFLY (AN/ APS -27) (PPI 
PRESENTATION) 

24.2.1 General Description 

Firefly 25 is a lightweight airborne radar system 
based on the noncoherent doppler phenomenon. It 
utilizes a mercury delay-line cancellation unit or a 
storage tube cancellation unit to eliminate ground 
clutter and to present only the moving vehicle echoes 
upon the plan position indicator [PPI] screen. Al- 
though the Firefly system arose to fulfill an urgent 
military need for the detection of moving vehicles 
at night or in foggy weather, other applications exist 
such as the detection of low-flying aircraft over land 
(anticollision radar), the detection of ship movements 
close to shore, drift angle determination, and in 
conjunction with the AN/APA-5 or AN/APQ-5 
bombing aids, the bombing or strafing of moving 
vehicle targets. In this connection it should be noted 
that moving vehicles provide positive identification 
of themselves by virtue of their motion, in contrast 
to ordinary radar echoes which must be identified 
as targets by their shape, intensity, or configuration 
with respect to other targets. See Figure 1, Chapter 
23 for a typical PPI photograph. 

From the tactical standpoint, Firefly is far superior 
to Butterfly in many respects: it can provide 360- 
degree coverage and explore the area in a circle of 
about 10 miles radius (314 square miles) once every 
3 to 6 sec depending upon the scanning rate ; with it 
the aircraft becomes an independent reconnaissance 
unit that need not be tied to a close control center; 



TRIGGER 


Figure 4. Firefly (AN/APS-27) moving vehicle de- 
tector. Simplified schematic block diagram. 


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FIREFLY (AN/APS-27) (PPI PRESENTATION) 


291 



TRIGGER 


Figure 5. Three-crystal cancellation unit. Simplified schematic block diagram. 


VIDEO 

INPUT 


10 MC 

OSCILLATOR 


10 MC 

MODULATOR 


A 


~7 


CHANNEL GAIN 
EQUALIZER 


10 MC 


10 MC 


ATTENUATOR 


UNDELAYED CHANNEL 


DELAY 

LINE 


UNDELAYED 

AMPLIFIER 


nFTFirmR 







DELAYED 

AMPLIFIER 


DETECTOR 



CANCELLED 

VIDEO 

AMPLIFIER 



0.5/i 

WAD r 

SEC 

DELAYED CHANNEL 

DETECTOR 

v Hn L/uLMI 


i 


TO PPI 


MODULATOR 

TRIGGER 


DOUBLE 

GATE 

GENERATOR 


AMPLIFIER 


TRACKING PULSE 
AMPLIFIER 


COINCIDENCE 

CIRCUIT 


TRACKING 

PULSE 

GENERATOR 


2000 - 
AUDIO 

OSCILLATOR 


D-C CONTROL VOLTAGE 
TIMING CHANNEL 


Figure 6. Two-crystal cancellation unit (automatic temporal and amplitude cancellation). 


it shows up entire lines of vehicles thereby providing 
characteristic outlines of the roadways and aiding in 
identifying priority targets; and, as mentioned above, 
it provides a means of immediate, independent, 
offensive attack upon the vehicles. Firefly is prob- 
ably not so sensitive to slowly moving vehicles as 
Butterfly, and there is no means for telling some- 
thing about the type of vehicle by examining the 
quality of the note as in Butterfly. However, the 


latter feature could easily be incorporated in Firefly 
if it were desirable. 

24.2.2 Technical Description 

The Firefly system is essentially a standard radar 
whose plan position indicator has a cancellation unit 
connected in series with the video lead to the PPI 
scope. A block diagram of the components of the 
system is shown in Figure 4. The following is a brief 


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292 


AIRBORNE MOVING TARGET INDICATOR SYSTEMS 


description of the components of the Radiation 
Laboratory experimental model. 

Radar System Characteristics 

The transmitter-receiver is similar electrically to 
that in the AN/ APS- 10. A 2J42 magnetron furnishes 
approximately 6 kw peak power output at a recur- 
rence frequency of 2,000 c, pulse duration 0.8 jusec. 
The power supply contains additional filters to elimi- 
nate modulation effects caused by hum. 

The receiver is a modified AN/APS-10 strip, to 
which instantaneous automatic gain control is added 
to increase the dynamic range. This results in a 
linear type of receiver rather than the linear-loga- 
rithmic type which is theoretically more desirable 
but considerably more complicated, bulky, and 
power-consuming. 

The antenna system utilizes standard X band r-f 


components and an AN/APS-15 cosecant-squared 
parabola. The standard AN/APS-15 sector-scan con- 
trol box provides 360-degree scanning or limited 
sector scanning as desired. 

The entire system, including the cancellation unit, 
presentation unit and power supply weighs approxi- 
mately 200 lb installed in an aircraft. 

Cancellation Unit (Mercury Delay-Line Type) 

During the course of the development of the sys- 
tem, three different types of cancellation units were 
built and tested. Two of them contain mercury delay 
lines (fixed delay) and the third a storage tube. 
Mercury provides a fixed delay of 17.52 n sec per 
inch of mercury at 20 C with a temperature vari- 
ation of +0.0052 jusec per inch per degree centigrade 
(mercury in steel). The delay time of both mercury 
units is 500 ^sec. 


DELAYED CHANNEL CANCELLED- VIDEO UNDELAYED CHANNEL 

AMPLIFIER AMPLIFIER AND LIMITER AMPLIFIER 



TEMPORAL MODULATOR OSCILLATOR - 

CANCELLATION TRIGGER MODULATOR 


B 



500 USEC 
DELAY LINE 


Figure 7. Two-crystal Firefly cancellation unit, experimental model, showing the mercury delay line (underneath) 
and the components of the cancellation unit on top of the chassis. The automatic amplitude equalizing circuit is not 
shown. 


| 


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w 



FIREFLY (AN/APS-27) (PPI PRESENTATION) 


293 



Figure 8. Double channel folded mercury delay line 
for AN /APS-20 (Cadillac). The right-hand pair of end- 
cells (containing the quartz crystals) are the input and 
output of the video delay channel which is folded so 
that the supersonic waves traverse a total path equal to 
four times the length of the unit. The timing channel 
(left-hand pair of cells) is folded once, and a gating 
circuit eliminates alternate triggers from the radar 
modulation. 

The components of the 3-crystal cancellation unit 
are shown in Figure 5. In this unit, video signals from 
the radar pass into a 10-mc modulator and divide 
upon emerging, part of the power going into the 
500-jusec line and the other part directly to the un- 
delayed channel. The delayed video is picked up by 
crystal No. 2 in the mercury delay line, then ampli- 
fied and passed into a detector. The delayed signal 
unites with the undelayed signal from the succeeding 
transmitted pulse at the output of the two detectors 
which are connected back-to-back so as to produce 
cancellation of video signals of equal amplitude. 
Amplitude-modulated signals are not cancelled and 


therefore can pass on to the cancelled- video amplifier, 
limiter, and thence to the PPI. In this way, only 
moving-target signals get through to the PPI. 

Synchronization of the PRF so that the time 
interval between successive transmitted pulses cor- 
responds exactly (to within 0.02 jusec) to the time 
delay in the line is accomplished in the timing chan- 
nel loop which contains a trigger generator, crystals 
1 and 3, the intervening mercury column, and the 
8-mc amplifier and detector. The timing pulse origi- 
nates in a blocking oscillator in the trigger generator, 
and passes into crystal No. 1 which it shock-excites 
to a frequency of about 8 me. The shock wave travels 
down the line, strikes the 45-degree reflector block, 
passes into crystal No. 3, is amplified, detected, and 
triggers off the next pulse of the blocking oscillator in 
the trigger generator. The time delay between trig- 
gers is adjusted by moving the 45-degree block to 
produce best temporal cancellation of fixed echoes. 
The modulator trigger comes from the trigger gen- 
erator. 

Amplitude cancellation of signals from the delayed 
and undelayed channels is accomplished by adjusting 
an attenuator placed in the undelayed channel. The 
bandwidth of the undelayed and delayed channels 
is about 3 me. The bandwidth of the mercury line 
is 4 to 5 me. 

The components of the 2-crystal, automatic tem- 
poral cancellation unit are shown in Figure 6. The 
operation of the 10-mc modulator, delayed channel, 
undelayed channel, back-to-back detectors, can- 
celled-video amplifier and limiter is identical with 
that of the corresponding components in the 3-crystal 
unit. 

The timing-channel loop, however, is entirely dif- 
ferent. Its function is to provide a control voltage for 
the 2,000-c timing audio oscillator, thereby control- 
ling its frequency so that the recurrence interval will 
correspond exactly to the video delay introduced by 
the mercury line. The 2,000-c sine wave originating 
in the timing oscillator is sharpened by the trigger 
generator into a l-^sec “tracking pulse.” This pulse 
is fed into the double-gate generator and also through 
a short video delay (approximately 0.4 //sec) to the 
10-mc modulator. The tracking pulse modulates the 
10-mc carrier and travels down the line to the second 
crystal, then to detector No. 3, and through the 
tracking pulse amplifier to the coincidence circuit. 
This process requires approximately 500.5 nsec total 
(for a 500-jusec mercury delay line). Meanwhile, the 
next tracking pulse, 500 jusec later, generates the next 



294 


AIRBORNE MOVING TARGET INDICATOR SYSTEMS 



Figure 9. Absorbing end-cells with saw-tooth crystal support (A). The interstices between teeth are filled with 
mercury as shown in B (hermetically sealed type). Construction of unit is shown in C. 


double gate, the center of which lies at 501.0 Msec 
(since the two gates are separated by 1 nsec and each 
one is 1 Msec wide) . The center of the tracking pulse 
is also at 501.0 Msec (since the leading edge arrives 
at the coincidence circuit at 500.5 jusec and the pulse 
itself is 1.0 /zsec wide). Consequently, the tracking 
pulse from the delay line lies equally in the two gates. 
As long as this condition remains unchanged, the d-c 
voltage output of the coincidence circuit remains un- 
changed, keeping the frequency of the timing oscil- 
lator constant. However, if the tracking pulse tends 
to lie more in one gate than the other, the d-c error 
voltage generator changes so as to alter the frequency 
of the audio oscillator thereby shortening or length- 
ening the recurrence time interval. In this way the 
recurrence interval is maintained identical with the 


video delay in the line. The variable delay section 
inserted between the signal generator and the modu- 
lator provides the means for initially compensating 
for circuit delay time. The modulator is triggered by 
a pulse from the double-gate system. Although the 
circuit is complicated, it successfully eliminates the 
problem of maintaining temporal cancellation. 

The amplitude equalizing circuit suggested in Fig- 
ure 6 serves to analyze the cancelled video of the 
tracking pulse. If amplitude cancellation is imperfect, 
there will be a residual signal, plus or minus, depend- 
ing on the relative gain of the delayed and undelayed 
channel amplifiers. The function of the equalizer is 
to provide an automatic gain control voltage on the 
undelayed channel amplifier thereby equalizing the 
amplitudes at all times. 


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FIREFLY (AN/APS-27) (PPT PRESENTATION) 


295 


2.2 K +285 -300V 



6 AC 7 6AC 7 6AC7 

Figure 10. Low capacity input pickup amplifier for storage tube cancellation unit.. 


Two types of mercury delay lines are illustrated in 
the photographs in Figures 7 and 8. The attenuation 
of the 500-Aisec line constructed of %-in. tubing is 
about 55 db. Reflections from the crystal faces at the 
ends of the line may be absorbed either in special 
mercury-backed saw-toothed supports for the crystals 
(see Figure 9) so that the power contained in the 
second traversal reflection of echoes is insignificant, 
or else in the line itself by introducing attenuation or 
varying other design parameters, for example, carrier 
frequency and line diameter. Cancellation ratios of 
uncancelled video signals to the same signals when 
cancelled range from 30 to 50 db in well-designed 
mercury delay cancellation units. 

Solids, including quartz and aluminum, have also 
been utilized in delay lines. Further development of 
solid delay lines and end-cells is in process. 

Cancellation Unit (Storage Tube Type) 

As explained in Chapter 23, any fixed type of de- 
lay line has the disadvantage that it must establish 
the recurrence frequency of the system; on the other 
hand, the storage tube allows the recurrence fre- 
quency to be established by other means. One oper- 
ational advantage of this is that the PRF can be 
varied, thereby eliminating blind speed regions. A 


second is that hum effects can be eliminated by syn- 
chronizing PRF with the a-c power frequency. A 
third is that the storage tube type may be somewhat 
lighter than the mercury delay line, thus making 
it more suitable for airborne use. The phenomenon of 
image storage on the cathode-ray tube face is de- 
scribed in Section 23.3.2, under “ Storage Tube 
Type” and the major components of a storage tube 
cancellation unit are illustrated in Figure 5, Chapter 
23. The heart of the system is the pickup amplifier 
which employs a feedback input circuit that effec- 
tively neutralizes the input capacity of the pickup 
plate, thereby providing reasonably good bandwidth 
in the high-impedance input stage, coupled with 
good signal-to-noise characteristics. The circuit dia- 
gram of the experimental model of the pickup ampli- 
fier is shown in Figure 10. 

In the present stage of development the maximum 
length of sweep permissible on the 5-in. tube seems 
to be about 50 n sec. A longer sweep results in exces- 
sive reduction in resolution of adjacent signals. If a 
range greater than 5 miles is desired it would be 
necessary to employ a spiral trace or step trace on 
the A scope. Cancellation ratios of about 20 to 30 db 
are secured with the present type of tube and pickup 
amplifier. Best results are obtained with a low in- 
tensity, well-focused beam. 


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296 


AIRBORNE MOVING TARGET INDICATOR SYSTEMS 



Figure 11 A. Firefly. Vehicles on Newburyport Turnpike. Range circle of 5 miles. Details of the ground are 
“painted” in by mixing uncancelled with the cancelled video of the PPI. Heavy land painting. 


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FIREFLY (AN/APS-27) (PPI PRESENTATION) 


297 



Figure 11B. Firefly. Vehicles on Newburyport Turnpike. Range circle of 5 miles. Details of the ground are 
“painted” in by mixing uncancelled with the cancelled video in the PPI. Medium land painting. 


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298 


AIRBORNE MOVING TARGET INDICATOR SYSTEMS 



Figure 12A. Worcester Turnpike, approaching Boston. Sweep range about 4 miles — view showing multiplicity of 
moving targets seen on the approach to Boston. 


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FIREFLY (AN/APS-27) (PPI PRESENTATION) 


299 



Figure 12B. Worcester Turnpike, approaching Boston. Sweep range about 4 miles — another view showing multi- 
plicity of moving targets seen on the approach to Boston. 


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300 


AIRBORNE MOVING TARGET INDICATOR SYSTEMS 


Presentation 

The photographs in Figures 1 and 3, Chapter 23, 
and Figures 11 and 12 of the present chapter show 
what may be expected from the Firefly system. High 
contrast presentation with low receiver gain is illus- 
trated in Figure 1, Chapter 23. 

In Figure 1 1 the effect of mixing cancelled and un- 
cancelled video signals is illustrated. The uncancelled 
video “paints” a faint background of the terrain 
upon the tube face, thus aiding in navigation and in 
target identification. Even better results would have 
been obtained with a “duotone” or “three-tone” 
type of PPI in which the background painting is 
maintained at a uniform low level by a separate 
limiter on the uncancelled video. 

Operation 

Except for the additional function introduced by 
the cancellation unit, the operation of the Firefly 
system is only slightly more critical than that of a 
standard search radar. The cancellation unit requires 
two principal adjustments: temporal cancellation and 
amplitude cancellation. Temporal cancellation is 
achieved by minimizing the leading or trailing edge 
“spikes” on fixed echoes by fine adjustment of the 
PRF, and amplitude cancellation by minimizing 
amplitude noncancellation with the undelayed chan- 
nel gain control. As in the case of Butterfly, it is 
recommended that an echo box be incorporated in 
the airborne equipment to allow the radar operator 
to test, at the flick of a switch, the overall perform- 
ance of the set. 


24.2.3 Performance 

Except for reliability, the performances of the 3- 
crystal model (Model I) and the 2-crystal model 
(Model II) experimental cancellation units were not 
appreciably different. As may be seen from the photo- 
graphs, vehicles are easily visible out to 10 to 12 
miles ahead and 5 to 8 miles abeam for an aircraft 
flying between 2,500 to 5,000 ft altitude. Noncancel- 
lation wings are apparent in Figure 3, Chapter 23, 
where the receiver gain was high. Noncancellation 
also appears at large beam tilt angles (greater than 
30 degrees). On one occasion, a small aircraft flying 
at 1,000 ft was seen out to the limit of the sweep (3 
miles, at the time) when the Firefly aircraft was at 
3,500 ft. The flight tests conducted at the Radiation 
Laboratory have not adequately explored the po- 
tentialities and limitations of the Firefly device. 


24.3 AIRBORNE MOVING TARGET 
INDICATOR FOR CADILLAC (AN/APS-20) 
(HIGH-POWER, LONG-RANGE 
AIRCRAFT DETECTION) 

24.3.1 General Description 

The pulsed coherent doppler principle was ap- 
plied to the high-powered “Cadillac” airborne early- 
warning radar to fulfill a definite military need. 28 The 
Cadillac system is so powerful that, when used over 
water, sea clutter obscures all aircraft out to ranges 
of 30 to 50 miles, depending upon the roughness of 
the sea, altitude of the radar and target aircraft, and 
other factors. The airborne moving target indicator 
[AMTI] system cancels the sea clutter and renders 
the Cadillac radar useful for aircraft detection at 
reasonably close ranges. 


24.3.2 Technical Description 

The Cadillac system 281 26-28 operates at a pulse 
recurrence frequency of 1,000 c. It is an S band sys- 
tem. A hard-tube modulator with well-filtered power 
supplies supplants the original spark-gap modulator. 
A simplified block diagram of the components of the 



Figure 13. AMTI coherent pulsed doppler system. 
Simplified schematic block diagram. 


AMTI portion of the system is shown in Figure 13. 
The general principles involved are explained in 
Section 23.2.1 under “Coherent Pulsed-Transmission 



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CONCLUSION 


301 



Figure 14. Velocity cancellation computer for a 10-cm airborne coherent pulsed doppler system. The shifting 
frequency, fe can be varied from 0 (abeam) to ±3.5 kc (fore and aft) corresponding to a maximum ground speed of 
about 400 mph. 


Doppler Method.” The transmitted r-f pulse from 
the magnetron beats with the stable local oscillator 
[STALO] and starts up the 30-mc coherent oscillator. - 
[COHO] in a phase which depends upon the combi- 
nation of the STALO and r-f phases. The returning 
echo beats with the STALO and produces i-f whose 
phase depends upon the combination of the phases of 
STALO, r-f, and phase due to the range of the target. 
If the target remains at fixed range, then the phase 
of the i-f bears a fixed relationship with respect to 
the COHO. When the i-f and COHO are combined 
in the detector, the resulting video signal will be up 
or down, and its amplitude from pulse to pulse will be 
fixed. The phase caused by moving targets will 
change from pulse to pulse, and the video will show 
amplitude modulation. For the airborne system the 
fixed targets are also moving, and their motions can 
be cancelled out by introducing the proper phase 
shift in the starting phase of the COHO from pulse 
to pulse. This is accomplished in the computer box 23 
where, by an ingenious system of high-frequency 
carriers and single-side-band amplifiers and de- 
tectors, the COHO, at 30 me is mixed with an audio 
frequency f e of from 0 to 3,500 c to produce a new 
frequency equal to 30 me + f e . A block diagram of the 
phase shifter portion of the computer box is shown 
in Figure 14. 

The present plan for the AMTI system calls for 


utilizing noncoherent pulsed doppler out to 10 miles 
range and the coherent system thereafter. 

Modulator triggering is obtained from the cancel- 
lation unit, which utilizes a double-delay line, the 
shorter section of which is the time element in the 
triggering circuit. (See Figure 8.) 

24.3.3 Performance 

At the time of this writing, the complete AMTI 
system had not yet been flown, although all the com- 
ponents had been laboratory tested and all portions 
of the systems except the coherent oscillator had 
been flight tested. Figure 15 shows a representative 
PPI photograph taken on an early flight, using non- 
coherent pulsed doppler detection. Single aircraft 
have been seen out to about 70 miles over land with 
the system. 

24.4 CONCLUSION 

It is the writer’s opinion that the airborne moving 
target detection device can provide over land the 
same type of valuable aerial radar reconnaissance 
that is now provided over water by airborne ship- 
search sets. When employed against aircraft, the 
system could be used to provide the information for 
an airborne fighter control center. In long-range 


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302 


AIRBORNE MOVING TARGET INDICATOR SYSTEMS 



Figure 15. AMTI. Range markers are at 5-mile intervals. 


bombing missions, bombers and their fighter escort or to evade them, whichever seemed to be the more 
could be controlled and coordinated as a single desirable. 

tactical unit by an accompanying airborne control Like tanks, gun, and other implements of war the 
center. Enemy fighters could be detected at great airborne moving vehicle detector probably has little 
distances and adequate preparations made to meet peacetime application. 


RESTRICTED 



GLOSSARY 


AEW. Airborne early warning (Cadillac). AN/APS-20 plus 
other components. A 10-cm radar system to be used pri- 
marily as cover for naval task forces. Radar information 
from an airplane is relayed to the aircraft carrier. 

AFC. Automatic frequency control. 

AGC. Automatic gain control. 

AGL. Airborne gunlaying. Any completely automatic airborne 
gunlaying system. 

AGS. Airborne gun sight. A manually operated gun-pointing 
system, in which the operator tracks from a scope indication. 

AI. Aircraft interception. A general designation for systems for 
detecting one aircraft from another. 

AIA. A 3-cm AI system for carrier-based fighter aircraft. 

AIBR. Acceleration integrator bomb release (refers to toss- 
bombing). 

Aided Tracking. A combination of displacement tracking and 
rate tracking, that is, the operator has direct single-knob 
control of both the position and velocity of some reference 
line, such as the sight line or the gun line. 

Amplidyne. A d-c generator in which the response of the out- 
put voltage to changes in field excitation is very rapid; used 
extensively as part of a servo follow-up system. 

AMTI. Airborne moving target indicator. 

A-N Beam. Radio beacon to guide aircraft. 

AN/. Indicates joint Army-Navy designation for a system. 

AN/APA-. Designates an attachment to an airborne radar 
system. 

AN/APA-5. LAB. An auxiliary radar bombsight to be used 
with a search radar such as AN/APS-1, -15, -30, especially 
for low-altitude bombing. 

AN/APA-16. Automatic low-altitude bombing attachment 
for search radars. 

AN/APA-40 (40-A). Micro-H Mk II. A delay unit for use with 
AN /APS- 15 or AN/APQ-13. 

AN/APA-46. Nosmo. An attachment for bombing radars de- 
signed to provide synchronous tracking, using the Norden 
sight. 

AN/APA-47. Visar. A system similar to AN /APA-46 (Nosmo) 
in which the visual bombardier performs the radar bombing 
also. 

AN/APG-. Designates airborne radar (“pulsed”) gunlaying 
or gun-sighting systems; also includes rocket sighting sys- 
tems. 

AN/APG-1. A 10-cm AI and AGL system. 

AN/APG-2. A 10-cm AI and AGL system. 

AN/APG-3. A 3-cm gunlaying radar. 

AN/APG-4. Sniffer. A 73-cm FM system for automatic bomb- 
release at altitudes up to 400 ft. 

AN/APG-5. A 12-cm ARO system. 

AN/APG-13 (13A). Falcon. A 12-cm range-only radar for 
75-mm cannon and rocket fire against water targets and 
isolated land targets. 

AN/APG-13B. Vulture or Overland Falcon. A 10-cm range- 
only conical-scan radar for cannon or rocket fire against 
land targets. 

AN/APG-15 (15A, 15B). A 12-cm conical-scan AGS system. 


AN/APG-16. A 3-cm gunlaying radar, similar to AN/APG-3. 

AN/APG-19. A 3-cm gunlaying system. 

AN/APG-21. Terry, Pterodactyl, or automatic Vulture. An 
automatic air-to-ground range-only radar, primarily for 
rocket fire. 

AN/APN-1. A 68-cm FM radio altimeter, usable up to 4,000 ft. 

AN/APN-19A. Airborne respondor beacon. 

AN/APQ-5. LAB. A low-altitude bombing system. 

AN /APQ-7. Eagle. A 3-cm bombing radar. 

AN/APQ-13. H2X. A 3-cm high-altitude bombing and naviga- 
tion radar for use over land, similar to AN /APS-15. 

AN /APS-. Designates an airborne search or interception radar 
system; frequently adapted for bombing. 

AN/APS-2. ASG. A 9-cm ASV and search radar. 

AN/APS-3. A 3-cm medium- and low-altitude bombing radar. 

AN/APS-4. ASH. A 3-cm ASV, AI and search radar for car- 
rier-based aircraft. 

AN/APS-6 (6A). A 3-cm search and interception radar, de- 
veloped from AIA. Designed for carrier-based night fighters. 

AN/APS-10. A 3-cm lightweight search and navigation sys- 
tem. 

AN/APS-15, 15A. H2X. A 3-cm high-altitude bombing and 
navigation radar for use over land. 

AN/APS-1 6. A 57-cm tail-warning radar. 

AN/APS-1 9. A 3-cm search and interception radar. 

AN/APS-20. AEW or Cadillac. See AEW. 

AN/APX-15. Ella. Identification system (for B-29), depend- 
ing upon propeller modulation. 

AN /ASG- 10. A nonradar toss-bombing system. 

AN/CPN-2. Ground beacon for precision navigation. 

AN/CPN-6. An X band ground respondor beacon. 
AN/CPS-1. See MEW. 

AN /CPS-6. V beam. S band early w arning and GCI ground 
radar system. 

AN /PPN-1,2. VHF respondor beacons for paratroops. 

AN/TPN-1. VHF transportable respondor beacon. 

AN/UPN-1,2,3,4. Ultra-portable respondor beacons. 

Angle, drift. The angle, in the horizontal plane, between the 
longitudinal axis of an airplane and its path relative to the 
ground. 

Angle of attack. The angle (measured in the vertical plane 
through the axis of the fuselage) between the line of flight 
of an airplane and some fixed reference line in the airplane, 
such as the line determined by the leveling lugs, the bore- 
sight datum line, or the zero-lift line. It varies with the 
speed, weight, and dive angle. 

Angle off. The angle between the line of flight of an airplane 
(usually a bomber) and the line joining it to an aerial target; 
sometimes measured from the nose and sometimes from the 
tail. 

Antenna. A conductor or system of conductors for radiating 
or receiving radio waves. A radar antenna includes the 
transmission-line feed or waveguide feed, the radiating 
elements proper, and the reflector. 

Antenna, driven. An antenna w hich receives its power from 
the transmitter through the transmission line. 


304 


GLOSSARY 


Antenna gain. A measure of the degree to which the radiation 
pattern is unidirectional; the ratio of the power per unit 
solid angle in the optimum direction to that from a source 
of equal power radiating isotropically. 

Antenna, parasitic. An antenna which is not driven, but 
receives its current by induction from one or more other 
antennas. 

Antenna pattern. The angular distribution of radiated power 
from the antenna assembly. 

Antenna, yagi. Consists of a reflector behind and a series of 
“ directors,” shorter than half a wavelength, which are 
placed in a row in front of a driven antenna. A narrow 
beam of radiation is produced, with the maximum radiation 
in the direction of the line of centers of the antennas (end- 
fire parasitic array). 

AR. Aircraft rocket. 

Arma resolver. A device used to perform vector addition of 
a-c voltages. 

ARO. An airborne range-only radar system; includes S band, 
X band, and FM systems. 

ASB. A 60-cm Navy radar for surface search by carrier-based 
aircraft. 

ASE. VHF airborne radar for surface search. 

ASG. AN/APS-2. 

ASH. AN/APS-4. 

ASV. A radar system for detecting and homing on a surface 
vessel from the air. 

AS VC. A 170-cm ASV system. 

ATR. Anti transmitter-receiver. A gaseous discharge type of 
switch which when fired leaves the magnetron matched to 
the transmission line, but when it has recovered, presents 
a mismatch for echoes which return toward the magnetron, 
thus forcing them through TR to receiver. 

Attenuation. Attenuation of a w r ave is the decrease in ampli- 
tude with distance along a transmission line in the direction 
of wave propagation, when the amplitude at any given 
place is constant in time. 

Attenuator. A device for controlling the amplitude of a 
signal. There are two types of r-f attenuators, cutoff 
(operating on the principle of a waveguide below cutoff), 
and dissipative (series resistance, or shunt conductance). 

Autosyn. A synchro device like the selsyn (q.v.). 

AVC. Automatic volume control. 

Bandwidth. The difference between specified frequencies 
(in cycles per second) of a frequency band; usually these are 
the half-power points in the frequency spectrum. 

Base line. The horizontal or vertical line formed by the 
movement of the sweep on a cathode-rav tube with de- 
flection-modulated presentation, for example, type A. 

Beacon. An interrogated radar transmitter by means of 
which an aircraft can determine azimuth and range with 
respect to the location of the beacon. 

Be am width. The angle between the half-power intensities of 
the radiation of an antenna. 

Bias. A potential difference between the electrodes of a 
vacuum tube; usually applied to that between cathode and 
a grid. 

Bias error. A constant error as opposed to a random error. 

Black maria. A radar system for the identification of friendly 
aircraft, designed to be used with AEW. 


Blocking oscillator. An oscillating vacuum-tube circuit 
containing a vacuum tube and a transformer, which pro- 
duces pulses at a predetermined recurrence frequency. It 
may be free running or under control of a synchronizing 
voltage. 

Bomb-release circle. For a given airspeed and altitude the 
locus of points at which a bombardier can release his bombs 
and hit the target providing he has the correct heading. 
This term is also applied to the electronic plot of such 
points on a radar scope. 

B scope. Signal appears as bright spot, with azimuth angle 
as horizontal coordinate and range as vertical coordinate. 

B' scope. Similar to B scope, with elevation vertical and 
range horizontal. 

BUPS. AN/UPN-1, -2. 

BUPX. AN/UPN-3, -4. 

Butterfly. Radar for detection of moving vehicles by an 
aircraft. 

c. Cycles per second. The symbol ~ is also used for this 
term. 

Cadillac. See AEW. 

Cancellation unit. A delay unit in which signals returned 
from nonmoving targets are cancelled out. 

Cathode-ray tube (CRT, oscilloscope, scope). A vacuum 
tube in which an electron beam is deflected by means of 
electric or magnetic fields. From the deflection, as observed 
on the face of the tube, the instantaneous values of the 
actuating voltages can be learned. 

Central-station computer. An airborne gun-directing sys- 
tem which operates turrets by remote control. 

CIT. California Institute of Technology. 

Clamp. To hold the base of waveform or pulse to a given po- 
tential or current value. 

Clutter. Radar signals from ground, sea, or other reflectors, 
appearing in an oscilloscope indication and interfering with 
observation of the desired target signals. 

COHO. Coherent oscillator. 

Coincidence Circuit. A circuit which transmits a pulse only 
when two or more input pulses coincide in time. 

Conical scan. A system of scanning in which the axis of 
symmetry of the power beam describes a cone, usually of 
small angle. It is used when the angular position of a target 
must be known accurately. 

Corner reflector. A metallic or metal-coated structure re- 
sembling the corner of a cube, particularly effective in re- 
flecting a radar beam. 

Cosecant-squared beam. A radar beam pattern designed to 
give uniform signal intensity for echoes received by air- 
borne radars from distant and nearby objects. The beam 
intensity varies as the square of the cosecant of the eleva- 
tion angle. 

Countermeasures. Measures to combat enemy radar, such 
as jamming, window, antiradar paint, Schnorkel. 

Crossover. The line about which the power beam from a 
conical-scan antenna revolves; also the relative power in 
the transmitted beam along that line in the antenna pattern. 

Cross trail. See Figure 2, Chapter 6. 

C scope. Presentation in which the signal appears as a bright 
spot with azimuth as horizontal coordinate and elevation 
as vertical coordinates. 


N 


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m 


GLOSSARY 


305 


CW. Continuous wave. 

CXEH. A Navy beacon similar to AN/CPN-6, an X band 
ground respondor beacon. 

db. Decibel, a unit used to express a power ratio. The number 
of decibels equals ten times the logarithm to the base 10 of 
the ratio of the two powers, e.g. “3 db down” means a 50 
per cent loss of power. 

Decay constant. The time required for a quantity to decay 
to 1/e of its original value. See time constant. 

Delay. Refers to a delay in the passage of a current (or volt- 
age) from one part of the circuit to another. 

Delay line. An artificial transmission line which produces as 
output a duplicate of what was given to it a definite short 
time before. 

Detail part. An element of an assembly, such as condenser, 
resistor, choke. 

Director sight. In this the gunner controls the line of sight. 
As he tracks, the computer positions the guns; see dis- 
turbed-reticle sight. 

Dipole (antenna). Two metallic elements, each approxi- 
mately a quarter wavelength long, which radiate the r-f 
energy fed to them by the transmission line. 

Dish. Antenna reflector. 

Disturbed-reticle sight. A computing gunsight in which the 
gunner controls the gun line, and, as he tracks, the computer 
deflects the sight line from the gun line by the amount of 
the computed lead angle. 

“Ditch.” Abandon aircraft. 

Doppler shift. A shift in the frequency of a wave caused by 
the relative motion of the source and receiver. 

Drift angle. See angle, drift. 

Driven antenna. See antenna, driven. 

Drone. A pilotless aircraft. 

Duplexer. An assembly (containing a TR tube) which di- 
rects the received energy to the receiver and excludes the 
very much greater transmitted energy. This allows the same 
antenna and transmission line to be used for both sending 
and receiving. 

Duty cycle. Ratio of transmitter time on to repetition period, 
e.g., a l-/usec pulse repeated every 500/z sec would have a 
duty cycle of 1 /500. Duty ratio and duty are other terms 
for this. Duty factor is its reciprocal. 

Eagle. AN/APQ-7. 

Echo box. A high Q resonant cavity which receives r-f energy 
through a pick-up antenna during the transmitted pulse 
and reradiates this energy through the same antenna im- 
mediately after the pulse. The reradiated energy is picked 
up by the radar set. Since this energy from the echo box 
dies off exponentially, it will appear on an A scope indicator 
as a flat-topped pulse, resulting from the saturation of the 
receiver by the high energy return, followed by an ex- 
ponential curve. The time from the end of the transmitted 
pulse to the time that the echo box signal is lost in noise is 
called the “ringing time” of the echo box. The echo box 
may be used to test the overall r-f performance of the 
radar set, and if the echo-box pick-up is in the antenna 
beam, the form of the antenna pattern can be shown 
graphically on the PPI. 

Ella. AN/APX-15. 

E plane. The plane of the electric vector of a beam of radi- 
ated power. 


Eureka. Respondor beacon. 

Expanded gain. The addition of a small portion of the indi- 
cator sweep voltage to the receiver gain voltage. 

Exponential smoothing. A function x = x(t) is said to be 
exponentially smoothed when it is replaced by y = y{t) 

defined by the differential equation k — + y = x; see refer- 

dt 

ence 58 in the Part IV bibliography. 

Falcon. AN/APG-13A. 

Firefly. A modification of Butterfly giving a PPI presenta- 
tion. 

FM. Frequency modulation. 

Frame time. Time for a complete scan. 

Frequency pulling. A change in the frequency of a mag- 
netron or other oscillator caused by a change in the load 
impedance. 

Future range. When the aircraft in Figure 1, Chapter 21 is 
at A, the distance EC is the future range. 

Gain. A power ratio, usually referring to an amplifier. 

Gain, Antenna. See antenna gain. 

Gate. A square voltage pulse which switches a circuit on or 
off electronically. 

GCI. Ground-controlled interception. 

GEE. A British navigation and bombing technique. 

GEE-H. A beacon-bombing system based on GEE equipment. 

GPI. Ground position indicator. 

Ground range. The distance from a point on the ground 
directly beneath an aircraft to a ground target or ground 
radar. 

G scope. A type of indicator presenting a spot with wings, 
which grows as the target approaches; azimuth is the 
horizontal, elevation, the vertical coordinate. 

GTAP. Ground track aiming point. 

Gun-roll. A source of error in the computing of a lead by a 
gunsight arising from a neglect of one component of ro- 
tational motion. 

Gyro sight. A sight in which the angular rate is measured by 
a gyroscope. 

Harp Material. Antiradar coating; absorbs microwave 
frequencies. 

H bombing. Bombing with the use of a navigational system in 
which the aircraft interrogates two ground beacons to 
determine its position. 

Helipot. A helical potentiometer. 

HF. High frequency; 3,000 to 30,000 kc. 

H plane. The plane of the magnetic vector of a beam of 
energy. 

H2S. S band bombing and search radars. 

H2X. X band radars for bombing and search; includes 
AN/APS-15 and AN/APQ-13. 

HVAR. High velocity aircraft rocket. 

i-f. Intermediate frequency. In microwave radar, the i-f 
amplifiers are usually centered at 15, 30, or 60 me. 

IFF. Identification as friend or foe. Radar systems which 
usually “interrogate” and receive a coded response if 
the target is friendly. 

Impact prediction. Computation of bomb-release point. 

Indicator. A device for displaying a received radar signal; 
usually a cathode-ray tube, although a dial or drum re- 
corder may occasionally be meant. 

In-out switch. A switch for causing the range gates to un- 


RESTRICTED “ ? 


306 


GLOSSARY 


lock from a target signal and move to lesser or greater 
range. 

Interrogator. A transmitting IFF radar set. Signals from 
it are received by a transpondor, and the latter replies 
automatically, this repR in turn being received by the 
respondor. 

Intervalometer. A device for releasing a series of bombs 
at predetermined time intervals. 

Jinking. Evasive motion of an aircraft in a series of straight 
line segments connected by curves. 

J scope. A modification of type A in which the time sweep 
produces a circular range scale near the circumference of 
the CRT face. The signal appears as a radial deflection. 

K band. Refers to wavelengths around 1 cm. 

Killing drift. Changing the heading of an airplane to com- 
pensate for wind, so that its ground track will pass through 
a given target. 

LAB. Low-altitude bombing; AN/APA-5 and AN/APQ-5 are 
examples. 

Lead-computing sight. A gunsight which computes the angle 
between the bore axis of the guns and the line of sight which 
is necessary to obtain hits. 

LHTR. Lighthouse transmitter-receiver. 

Lighthouse tube. A small oscillator tube, so called from its 
appearance. 

LO. Local oscillator; a tube which produces a signal with a 
frequency near that of the transmitter. The LO signal is 
mixed with the echo to give a “beat” at intermediate fre- 
quency which is then amplified and detected. 

Lobe-switching. Directing an r-f beam rapidly back and 
forth between two or more positions. 

Local turret. An airplane gun turret controlled by an 
operator located in it. 

Longwave. Refers to wavelengths greater than 1 meter, as 
opposed to microwave radar. 

Loran. A hyperbolic grid system of long-range radio naviga- 
tion, in which the navigator observes the difference in 
arrival times of pulses from two known stations. 

L scope. A double A scope presentation for a double-lobe 
system. Deflections to the two sides of the time sweep 
indicate signals from upper and lower (or right and left) 
lobes. 

MAD. Magnetic airborne detector for submarines under water. 

Magnetron. A transmitter tube which produces the main 
pulse of ultra high frequency energy. The flow of electrons 
is controlled by an applied magnetic field instead of a grid. 

Major assembly. A self-contained combination of subassem- 
blies and detail parts, such as indicator unit, transmitter- 
receiver unit, power unit. 

me. Megacycles per second. One megacycle is a million cycles. 

MC-627. Automatic plotting table for close-support bombing. 

MEW. Microwave early warning, a 10-cm ground radar for 
long-range detection or control of aircraft (AN/CPS-1); 
allows continuous plotting, in range and azimuth, of multiple 
targets. 

Micro-H. H bombing with microwave radar systems. 

Microsecond, nsec, 10 -6 seconds. 

Microwave radar. Radar using wavelengths less than one 
meter. 

Mil. Abbreviation for milliradian, an angle of one-thousandth 
of a radian; one degree is 17.45 milliradians. 


Mil, artillery. An angle equal to 1/6400 of a circle; one 
degree is 17.78 artillery mils. 

Milliradian. See mil. 

MIT. Massachusetts Institute of Technology. 

Modulation. Varying the amplitude of the high-frequency 
signal according to a definite pattern. 

Modulator. Also called a pulser. The part of the radar set 
which sends the high-voltage pulse to the transmitter. This 
pulse, in turn, starts the oscillation of the transmitter, which 
emits microwave radiation. 

M scope. Modification of type A for range finding. The hori- 
zontal sweep is displaced vertically as in a step ; the position 
of this step can be adjusted by some controlling device so 
that it coincides with the signal, at which point the device 
registers range. 

MTI. Moving target indicator. 

Multivibrator. A form of relaxation oscillator, essentially a 
two-stage amplifier with feedback. It will oscillate of its 
own accord or through an external synchronizing voltage. 

Mush. A vague descriptive term associated with the phe- 
nomenon of angle of attack of an airplane. An airborne 
fixed gun is said to mush when its bore axis is elevated above 
the line of flight. 

MV. Multivibrator. 

MX-344. A bombing computer. 

NDRC. National Defense Research Committee. 

Noise. A random voltage appearing at the output terminals of 
a receiver with no impressed signal, if the amplifier has 
sufficient gain. On the A scope, noise appears as random 
spikes (“grass”) on the sweep line. It is caused by random 
motion of electrons in the grid circuit of the first amplifier 
tube, fluctuations in emission, shot noise at the plate, etc. 

Noise figure. The figure of merit for sensitivity of a receiver. 
Defined as the ratio of the input power to kTB (where k is 
Boltzmann’s constant, T the temperature in degrees Kelvin, 
and B the bandwidth in c) when the output signal power 
equals the output noise power. Noise figure is normally ex- 
pressed in decibels. 

Nosmeagle. Nosmo for Eagle. 

Nosmo. AN/APA-46. 

Oboe. A British bombing technique. 

Offset bombing. Bombing in which the bombardier (visual or 
radar) sights on an aiming-point different from the target. 

OSRD. Office of Scientific Research and Development. 

Own-speed sight. Same as vector sight. 

Palmer scan. A type of antenna scan for searching. 

Parasitic antenna. See antenna, parasitic. 

Pass-band. Range of frequencies passed by a filter. 

PDI. Pilot’s direction indicator. 

Phantastron. A precision delay circuit. 

Plane of action. The plane containing the fine of motion of 
an aircraft and the target. 

Plumbing. Wave guide and coaxial cable or transmission line, 
with fittings. 

Polyrod. Polystyrene plastic rod. 

Position firing. A rule-of-thumb procedure for use by an 
aerial gunner whose gun is equipped with a ring and post 
sight. The lead taken depends only upon the relative bear- 
ing of the target. 

PPI. Plan position indicator. Scope indication with circular 


i 


RESTRICTED 


GLOSSARY 


307 


sweep, showing ground objects in approximately correct re- 
lationship as on a map. 

Pressurize. The filling of the r-f line with air at a pressure 
greater than atmospheric. Its purpose is twofold: (1) to 
prevent breakdown of the components at high altitudes and 
(2) to protect against transmission losses caused by materials 
in the atmosphere, such as dirt and water. 

PRF. Pulse recurrence frequency. 

Probable error. A magnitude associated with the measure- 
ment of a quantity such that half of the errors are less and 
half are greater than the given magnitude. 

Proximity fuse. A fuse for shells, bombs, or rockets which 
sends out radio waves and explodes at a predetermined dis- 
tance from a target (YT fuse). 

Pulse. Refers to the emission of power for a short time, fol- 
lowed by a period of no emission; one of the fundamental 
characteristics of most radar. 

Pui.se shape. The graph of radiated energy as a function of 
time. 

Pulsed doppler shift or principle. See Part V. 

Pursuit course. A course in which a pursuer is continuously 
moving in the direction of the pursued; see Section 21.1.1 
for more complex modifications of this concept, such as 
lead pursuit, aerodynamic pursuit, and aerodynamic lead 
pursuit course. 

Q (of a resonant system). The Q of a specific resonance 
mode of a system is 2ir times the ratio of the energy stored 
to the energy lost per cycle, when the system is excited in 
this mode. A high Q circuit is lightly damped, has a small 
decrement, a sharp resonance peak, and a high selectivity. 
Q is a figure of merit. 

Radar. Abbreviation of “radio detection and ranging”; usu- 
ally refers to systems using ultra-high frequency waves, with 
the pulse technique. 

Radiation laboratory. In this book this designation is re- 
served for the MIT Radiation Laboratory which carried 
on radar research and development from 1940 to 1945, under 
the direction of Division 14, NDRC. 

Radome. A general name for radar turrets which enclose 
antenna assemblies. 

Range, future. See future range. 

Range mark. One of a series of spots or lines on a scope to 
indicate the range of target signals. 

Range wind. The component of the wind in the direction of 
the target. 

Rate end. A component of the Norden sight. 

Rate sight. A gunsight in which the lead is computed from 
the rate of tracking of the target. 

RC network. A circuit containing resistances and capaci- 
tances. 

RC-294. Plotting board for SCR-584. 

Receiver sensitivity. Related to the ability of a receiver to 
detect weak signals. It is measured by the noise figure (q.v.) 
in the case of microwave radar. 

Reflector, corner. See corner reflector. 

Responsor. See interrogator. 

r-f. Radio frequency. A general term for the frequency to be 
radiated, not confined to any specific limit. 

r-f head. A major assembly unit of a radar system which in- 
cludes the magnetron, duplexer, part or all of the receiver, 
and occasionally other parts. 


Ringing circuit. A circuit in which the oscillations die out 
slowly, as when a bell is rung. 

Ringing time. See echo box. 

RL. Radiation Laboratory. 

Saw-tooth sweep. A sweep in which the motion of the electron 
beam is controlled by a saw-tooth voltage wave, that is, the 
voltage rises slowly and linearly and then declines rapidly. 

S band. Refers to wavelengths of the order of 10 cm. 

Scanner. A device which directs the r-f beam successively over 
all points in a given space. 

SCI. Ship-controlled interception. Similar to GCI. 

Scope. Oscilloscope, cathode-ray tube. For the various types 
of scope presentations, see under the appropriate letters. 

SCR. Signal Corps radio set. 

SCR-520. A 10-cm airborne search and interception radar. 

SCR-540. A 155-cm airborne radar for detection of other air- 
craft. 

SCR-584. Mobile medium-range search and track radar, de- 
signed for antiaircraft fire control, and also applied to ground 
control of aircraft. 

SCR-695. A 160- to 191-cm transpondor. 

SCR-717. An airborne radar system for detection of surface 
vessels. 

SCR-718. A 68-cm pulsed altimeter for use up to 40,000 ft. 

SCR-720. A 10-cm airborne search and interception radar, 
especially for night fighters. 

SCR-729. An IFF interrogator-responsor. 

Second detector. The detector which converts i-f (30 or 
60 me) into video. 

Sector scan. Motion of the scanner reflector back and forth 
through a limited angle, instead of through 360 degrees. 

Selsyn. A self-synchronous motor or generator (autosyn, 
synchro; the latter name has been chosen by the Services). 
A means of making a shaft rotate by the same amount as 
another shaft at some remote position. 

Servo-amplifier. The amplifier of power impulses in a servo 
system. 

Servo loop. That collection of elements in a servomechanism 
which measures the error in the quantity to be controlled 
and applies a correction tending to reduce that error to zero. 

Servo system. A mechanical, frequently an electromechanical, 
system for transmitting accurate mechanical position from 
one point to another by electrical or other means. The posi- 
tion is corrected by feeding back an error signal. 

Shoran. Short range navigational system made up of two 
ground radars (AN/CPN-2) and one airborne set 
(AN/APN-3). 

Side lobe. A portion of the beam from a radar antenna other 
than the main lobe; usually much smaller. 

Sinepot. Sine potentiometer. 

Skid (of an airplane). Motion of an airplane in a direction 
different from that in which it is heading. 

Skywave. A radio wave reflected from the ionosphere; this 
occurs at frequencies less than 20 me. 

Slant range. Range from an aircraft to a ground target or 
radar; distinguished from ground range. 

Sniffer. AN/APG-4. 

Spinner. Rotating antenna assembly; a scanner. 

SS loran. Sky-wave synchronized Loran. 

Stability (of a sighting system). Stability exists when, if the 


V 


RESTRICTED 


308 


GLOSSARY 


gun is given a small quick jerk in some direction, the reticle 
is jerked in the same direction. 

Stabilize (as a scope or line of sight). To maintain a system 
in a desired orientation, in spite of motion of aircraft or ship. 

Stadiametric ranging. Determination of range to an airplane 
target by bracketing the image between optical markers in 
the sight, which then computes the range by the principle 
of similar triangles. 

Stalo. Stable local oscillator. 

Subassembly. A part of a unit assembly, replaceable as a 
whole, consisting of a combination of detail parts (q.v.), 
such as i-f amplifier section, voltage regulator. 

Sweep. The beam of electrons passing from the electron gun 
to the face of the CRT makes a point of light on the face of 
the tube. By proper voltage or magnetic control this point 
of light can be made to move in any direction. By making 
this motion rapid and continuous, the point of light becomes 
a line of light, and is called a sweep. 

Sweep circuit or generator. A circuit which produces at 
regular intervals an approximately linear or circular, or 

r other form of movement (sweep) of the beam of the cathodc- 

* ray tube. 

Synchro. Same as selsyn, autosyn. This designation is pre- 
ferred by the Services. 

Synchronization (of a bombsight). Establishment of the 
proper rate of motion of the bombing computer index so 
that the index tracks the target. 

Synchronization (of a gunlaying system). Establishment of 
the tracking of a target in range and angle by the gunlaying 
system. 

Terry. AN/APG-21. 

Test equipment. An assortment of instruments provided with 
a radar set to enable the maintenance man to determine 
accurately whether the set is performing properly in its 
various functions and to aid in locating improperly opera- 
ting components and in restoring them to proper condition. 

Thermistor bridge. A bridge with sensitive resistors whose 
resistance varies significantly with temperature. 

Time base. The sweep on an indicator tube begins at zero time, 
the instant that energy is transmitted and ends at a later 
predetermined time. It may be called a time base. Since 
time and distance are proportional in the radiation of the 
energy from its source, the distance of any signal on the 
sweep from the beginning of the sweep may be translated 
into units of geographical distance. In some circuits, the 
beginning of the sweep is delayed for a fixed or variable 
time after the firing of the transmitter. It is then known as 
a delayed sweep. 


Time constant. The time required for a variable which obeys 
an exponential law to change by a fraction I /e of the total 
change. 

TR. Transmit-receive tube; a TR box or, preferably, switch, is 
the assembly containing the TR. See duplexer. 

Trail. The vector giving the displacement of the actual point 
of impact of a projectile or bomb from the point where it 
would have hit if it had moved in a vacuum. 

Trajectory drop. The angle (in mils) between the line along 
which a projectile was fired and the line from the gun to the 
position of the projectile. 

Transpondor. A radar system which receives and replies to 
an IFF interrogator (q.v.). Also a similar system used as a 
radar beacon for navigational purposes. 

TRE. Telecommunications Research Establishment (British). 

Trigger pulse. A pulse which starts a cycle of operations. 

Tuning. The process of adjusting circuits to resonance with 

the frequency of a desired signal. 

UBS. Universal bomb sight. 

UHF. Ultra-high frequency (200 to 3,000 me). 

V-beam. AN/CPS-6. 

Vector (verb). To direct (an airplane) toward a moving 
target (military usage in aircraft interception). 

Vector sight. A gunsight which gives the lead as a constant 
times the sine of the angle off. The constant depends upon 
the own speed of the aircraft, altitude, and ammunition. 

VHF. Very high frequency (30 to 300 me). 

Video. Electrical form in which a returned radar echo is trans- 
mitted to the indicator to be made visible. 

Visar. AN/APA-47. 

V scope. See Figures 16 and 21, Chapter 20. 

Vulture. AN /APG-13B. 

Waveguide. A hollow pipe, usually of rectangular form, used 
as an r-f transmission line. The limits on the dimensions of 
the pipe are determined by the wavelength to be transmitted 
by the pipe, also by the shape of the pipe and the mode of 
transmission. There are other types of waveguides, such as 
solid dielectric cables, through which it is possible to trans- 
mit energy. Waveguides may be straight, twisted, curved, 
tapered, or flexible. 

Window. Chaff. Radar countermeasure, consisting of strips of 
metal foil or metal-coated paper, cut to a calculated size, 
dropped from an airplane. A small quantity of the material 
will reflect as much energy as an aircraft. 

Wind triangle. See Figure 8, Chapter 13. 

X band. Refers to wavelengths around 3 cm. 

Yagi antenna. See antenna, yagi. 


BIBLIOGRAPHY 


The bibliography for MARS is divided into five parts, 
•corresponding to the parts of the book. The bibliography list 
given for each part includes all the references cited in the texts 
of that part and also supplementary references for that part. 
The references within each part are arranged according to 
sources. NDRC reports are listed first. NDRC reports for 
MARS include both regular reports issued by Divisions 3, 4, 7, 
14 and the Applied Mathematics Panel and papers written by 
contractors to these divisions. Thus, regular Division 14 re- 
ports, Radiation Laboratory Reports, Radiation Laboratory 
Internal and Informal Reports, and The Radiation Labora- 
tory Series, formerly known as the Radiation Laboratory 
Technical Series and so cited in the present volume, are listed 
under Division 14, NDRC. Army, Navy, and Joint War and 
Navy Department Reports, American manufacturing com- 


pany Reports, British Reports, and Miscellaneous Books are 
the other source headings for the MARS bibliography. 

References that are general or summary in nature are marked 
with an asterisk (*). Such references are considered important 
by the writers of this book and are suggested as supplementary 
reading matter for the topics mentioned in MARS. 

Several abbreviations are used to shorten the bibliography. 
Those are: RL for Radiation Laboratory, NDRC for National 
Defense Research Committee, AMP for Applied Mathematics 
Panel, AMG-C and AMG-N for Applied Mathematics 
Groups at Columbia and Northwestern, respectively. 

Numbers such as Div. 14-253.3-MI indicate that the docu- 
ment listed has been microfilmed and that its title appears in 
the microfilm index printed in a separate volume. For access 
to the index volume and to the microfilm, consult the Army 
or Navy agency listed on the reverse of the half-title page. 


PART I 

Chapters 2-5 


NDRC Division 14 Reports 

DIVISION 14 REGULAR REPORTS 

1. U. S. Radar Survey Section 6, Test Equipment , Change 1 , 
Office of the Secretary, NDRC Division 14, May 15, 
1945. Div. 14-251 -Ml 


RL TECHNICAL SERIES a 

2. Propagation of Short Radio Waves , J. L. Law r son, Editor. 
2a. Ibid., Sec. 219. 

3. Microwave Duplexers, C. G. Montgomery, Editor. 

*4. Radar System Engineering , L. N. Ridenour, Editor. 

4a. Ibid., Chap. 12. 

4b. Ibid., Sec. 2.7. 

4c. Ibid., Sec. 2.4. 

*5. Radar Aids to Navigation, J. S. Hall, Editor. 

5a. Ibid., Chap. 6. 

6. Radar Beacons, A. Roberts, M. D. O’Day, Editors. 


ARMY AIR FORCES 

7. “Schnorchel Worked,” RADAR No. 10, June 30, 1945. 


JOINT ARMY AND NAVY REPORTS 

8. Meeting of Ad Hoc Committee of the Joint Radio Board, 
T AC-322 to T AC-333, F. R. Banks, Joint Radio Board 
Report JRB-20, Apr. 25, 1945. Div. 14-253.3-MI 

8a. Ibid., TAC-322. 

8b. Ibid., T AC-323. 

8c. Ibid., T AC-324. 


MISCELLANEOUS BOOKS 

11. Radio Engineer’s Handbook , Frederick Emmons Terman, 
McGraw-Hill, 1943. 

11a. Ibid., p. 811. 


PART II 


Chapters 6-13 


NDRC Division 4 Reports 

DIVISION 4 REGULAR REPORTS 

*1. Toss Bombing, Summary Technical Report, Division 4, 
Volume 2. 


McLean, Special Group on Toss Bombing, Ordnance 
Development Division, National Bureau of Standards, 
Report OD-TB-19, Aug. 31, 1944. Div. 14-329.17-M3 


ORDNANCE DEVELOPMENT DIVISION — 
NATIONAL BUREAU OF STANDARDS 

2. Equations for Toss-Bombing for the Horizontal Case, As- 
suming Acceleration Is a Function of Time, William B. 


3. Toss Bombing Trajectories, F. L. Celauro, D. Fisher, 
Ordnance Development Division, National Bureau of 
Standards, Report OD-OAG-32, Sept. 6, 1944. 

Div. 14-329. 17-M4 


a References to specific sections and chapters of the RL Technical Series may be incorrect because of revisions made after 


this bibliography was printed. 



309 


310 


BIBLIOGRAPHY 


4. Effect of Changing Integrator RC Ratio to Correct for an 

Error in Alignment of Sight with Line of Flight , William B. 
McLean, Special Projects Group, Ordnance Develop- 
ment Division, National Bureau of Standards, Report 
OD-SP-40, Oct. 26, 1944. Div. 14-329. 17-M5 

5. Analysis of Horizontal Range Error Resulting from Neglect 
of Pull-up Angle , S. H. Lachenbruch, Special Projects 
Group, Ordnance Development Division, National 
Bureau of Standards, Report OD-SP-45, Nov. 7, 1944. 

Div. 14-329.17-M7 

6. Use of the 100-Ft. Error Curves for Errors of Other 
Magnitudes , S. H. Lachenbruch, Special Projects Group, 
Ordnance Development Division, National Bureau of 
Standards, Report OD-SP-46, Nov. 8, 1944. 

Div. 14-329. 17-M8 

7. General Toss-Bombing Solution for the Case of a Non- 
Constant Acceleration , Including the Effect of the Pull-up 
Angle, Albert London, Special Projects Group, Ordnance 
Development Division, National Bureau of Standards, 
Report OD-SP-48, Nov. 3, 1944. Div. 14-329. 17-M6 

8. Relationship Among Important Angles in Toss Bombing 
Trajectories, S. H. Lachenbruch, Special Projects Group, 
Ordnance Development Division, National Bureau of 
Standards, Report OD-SP-49, Nov. 10, 1944. 

Div. 14-329. 17-M9 

9. Application of Toss Bombing Equipment to Torpedo 
Tossing, Albert London, Special Projects Group, 
Ordnance Development Division, National Bureau of 
Standards, Report OD-SP-56, Nov. 28, 1944. 

Div. 14-329. 17-M 11 

10. Correction of the Acceleration Integrator for Air Resistance, 
S. H. Lachenbruch, Special Projects Group, Ordnance 
Development Division, National Bureau of Standards, 
Report OD-SP-76, Jan. 12, 1945. Div. 14-329. 17-M 12 

11. Tables of New \p Functions and Other Related Quantities, 
C. F. Eve and Albert London, Special Projects Group, 
Ordnance Development Division, National Bureau of 
Standards, Report OD-SP-77, Jan. 15, 1945. 

Div. 14-329. 17-M 13 

12. New \f/-Card Design, Albert London and A. E. Will- 

goos, Special Projects Group, Ordnance Development 
Division, National Bureau of Standards, Report 

OD-SP-78, Jan. 17, 1945 Div. 14-329.17-M14 

13. Rocket-Tossing Theory, Albert London and C. F. Eve, 

Special Projects Group, Ordnance Development Divi- 
sion, National Bureau of Standards, Report OD-SP-90, 
Feb. 24, 1945. Div. 14-329. 17-M 15 

14. Exact Solution of Toss-Bombing Equations for Circular 

Pull-up, S. H. Lachenbruch, Albert London, and C. F. 
Eve, Special Projects Group, Ordnance Development 
Division, National Bureau of Standards, Report 

OD-SP-98, Mar. 23, 1945. Div. 14-329. 17-M 16 

15. Range Limitations Resulting from Approximations in 

Toss Bombing Equations, S. H. Lachenbruch, Special 
Projects Group, Ordnance Development Division, Na- 
tional Bureau of Standards, Report OD-SP-105, Apr. 
16, 1945. Div. 14-329. 17-M 17 

16. Range Wind Correction for Toss Bombing, Albert London 
and C. F. Eve, Special Projects Group, Ordnance De- 
velopment Division, National Bureau of Standards, 
Report OD-SP-107, June 5, 1945. Div. 14-329. 17-M 18 


17. ^-Function for Non-Constant Pull-Up Acceleration, C. F. 
Eve and Albert London, Special Projects Group, 
Ordnance Development Division, National Bureau of 
Standards, Report OD-SP-123, July 10, 1945. 

Div. 14-329. 17-M20 

18. The Effect of Sight Misalignment and Angle of Attack 
Variation, S. H. Lachenbruch, Special Projects Group, 
Ordnance Development Division, National Bureau of 
Standards, Report OD-SP-131, July 23, 1945. 

Div. 14-329. 17-M21 

19. The Elements of Toss-Bombing, Irvin H. Swift, Tech- 
nical Paper REI-TMD-115 Rev. 1, OEMsr-769, Uni- 
versity of Iowa, Nov. 17, 1944. Div. 14-329.17-M10 


NDRC Division 14 Reports 

RL REPORTS 

*20. SCR-584 Plotting Table System, Ernest M. Lyman, RL 
Report 595, OEMsr-262, July 3, 1944. Div. 14-265.3-MI 

*21. Flight Behavior of the Flux Gate and Gyrosyn Compasses 
and Their Effects on GPI, W. J. Tull, RL Report 712, 
OEMsr-262, Apr. 30, 1945. Div. 14-329. 142-M3 

22. AN/APG-5 ( ARO ) as a Terrain Clearance Indicator, 

Robert M. Whitmer, RL Report 908, OEMsr-262, 
Jan. 16, 1946. Div. 14-323.1 1-M5 

23. A Photographic Method for Assessment of Bombing Re- 

sults, G. F. Wheeler, RL Report 939, OEMsr-262, Feb. 
28, 1946. Div. 14-329.151-M2 

*24. SCR-584, Bombing with Modification Kit MC-627, 
G. D. Huff, RL Report 986. 

25. Theoretical and Experimental Study of Radar Ground 

Return, Roger E. Clapp, RL Report 1024, OEMsr-262, 
Apr. 10, 1946. Div. 1 4-264. 1-M6 

26. Final Project Reports on Project 811.0, G. D. Huff, RL 
Report 1077. 

27. Preliminary Handbook of Instructions for Supersonic 
Trainer AN / APQ-7-T1 ( Eagle Trainer), Walter R. 
Carmody, RL Report M-189, OEMsr-262, June 14, 1945. 

Div. 14-41 1.1-M2 

28. Preliminary Instructions on Modification Kit M C-627 for 

Radio Set SCR-584 , G. E. Brunette, RL Report M-220, 
OEMsr-262, May 1, 1945. Div. 14-329.16-MI 

29. Preliminary Instructions on Modification Kit MC-627 for 

Radio Set SCR-584 (Revised), G. E. Brunette, RL Re- 
port M-220B, OEMsr-262, Projects AC-106 and AC- 
239.05, Nov. 28, 1945. Div. 14-329. 16-MI 

*30. Preliminary Handbook of Operating and Maintenance 
Instructions for Model AN /APA-46 Aircraft Radar 
Equipment, Helen Wenetsky, RL Report M-227, 
OEMsr-262, Project AC-232.08, June 1, 1945. 

Div. 14-329. 12-M6 

31. Additional Modification, Calibration, and Plotting Pro- 
cedures for RC-204 Plotting Equipment, J. E. Ward, RL 
Report M-235, OEMsr-262, Feb. 18, 1946. 

Div. 14-255.3-M6 

32. Preliminary Operation and Maintenance Handbook for 
Release Point Indicator AN /ARA-17, J. D. Horgan and 
J. E. Ward, RL Report M-241, Nov. 1, 1945. 

Div. 1 4-265. 3-M5 


BIBLIOGRAPHY 


311 


33. Handbook of Maintenance Instruction for Indicator As- 

sembly AN/APA-53, W. R. Slaun white and R. L. 
Kellner, RL Report M-243, OEMsr-262, Project NA-178, 
Oct. 24, 1945. Div. 14-321. 14-M17 

34. Operating and Maintenance Instructions for Indicator for 

Rapid Scan System , Paul Jarmotz, RL Report M-248, 
OEMsr-262, Apr. 5, 1946. Div. 14-242. 12-M8 

*35. H2K Radar Displays , C. F. J. Overhage, RL Report 
S-44, OEMsr-262, Apr. 9, 1945. Div. 14-329. 12-M4 

36. The Manual Plotting System RC-305, J. W. Brean, J. E. 

Ward, and J. D. Horgan, RL Report S-62, OEMsr-262, 
Aug. 31, 1945. Div. 14-265.3-M4 

37. Operational Procedure for AN/APA-5, W. J. Deerhake 

and K. E. Schreiner, RL Report S-67, OEMsr-262, 
Oct. 26, 1945. Div. 14-329. 12-M9 

RL INTERNAL REPORTS AND INFORMAL 
MATERIAL 

38. Unified Radar Bombsight ( URBS ), E. B. Meservey, RL 
Internal Report Group 71.0, OEMsr-262, July 31, 1945. 

Div. 14-329. 143-M2 

39. Notes on the Rebecca-H System from Information Obtained 

at T RE- July, 1943, Arthur Roberts, RL Internal Report 
Group 71, Aug. 25, 1943. Div. 14-327.3-MI 

40. Flight Test of an Experimental Horn-Fed Antenna for 
H2X , R. C. Ottens and J. E. Woodward, RL Internal 
Report Group 91.3, Dec. 1, 1944. Div. 14-234.21-M8 

41. Proposal for Extending the Range of Shoran or M-H 
Beacon Coverage by Use of GPI, W. J. Tull, RL Internal 
Report Group 91.3, July 20, 1945. Div. 14-327.2-M2 

42. AN/APA-5 , Preliminary Report on 1,000-Foot Runs, 

RL Informal Report from July 18-22, 1944, Aug. 7, 
1944. Div. 14-253. 1-MI 

43. AN/APA-5, Preliminary Report on 5,000-Foot Runs, 

RL Informal Report from Aug. 4-12, 1944, Aug. 17, 
1944 (corrected Sept. 6, 1944). Div. 14-253. 1-M2 

44. AN/APA-5, Preliminary Report on 15,000-Foot Runs, 

RL Informal Report from Aug. 25-28, 1944, Sept. 5, 
1944. Div. 14-253. 1-M3 

45. Preliminary Manual on the SCR-584 Close Support 
System, Technical Operation, Employment and Mainte- 
nance, Ernest M. Lyman and R. W. Larson, ASB Ref. 3, 
Advanced Service Base BBRL/ABI-15, Jan. 5, 1945. 

Div. 14-265. 1-M3 

46. Notes on the European and Eastern Atlantic S. S. Lor an 

Systems, R. H. Woodward and W. Lees, BBRL 83, Apr. 
17, 1945. Div. 14-327. 1-M4 

RL TECHNICAL SERIES b 

47. Vacuum Tube Amplifiers, George Valley, Editor. 

47a. Ibid., Chap. 16. 

*48. Electronic Instruments , B. Chance, Editor. 

48a. Ibid., Sec. 810. 

48b. Ibid., Sec. 18.7. 

48c. Ibid., Chap. 18. 

48d. Ibid., Chap. 17. 

48e. Ibid., Part I. 


49. Microwave Receivers, George Valley, Editor. 

49a. Ibid., Chap. 9. 

*50. Microwave Antenna Theory and Design, H. M. James, 
Editor. 

50a. Ibid., Chap. 14. 

*51. Radar System Engineering, L. N. Ridenour, Editor. 

51a. Ibid., Sec. 3.6. 

51b. Ibid., Chap. 2. 

51c. Ibid., Sec. 12.4. 

*52. Radar Aids to Navigation, J. S. Hall, Editor. 

52a. Ibid., Sec. 1A.1. 

52b. Ibid., Chap. 3.3. 

52c. Ibid., Chap. 1A. 

52d. Ibid., Sec. 3.6. 

*53. Radar Beacons , A. Roberts, M. D. O’Day, Editors. 

53a. Ibid., Sec. 5.9. 

*54. Radar Scanners and Radomes, M. B. Karelitz, W. M. 
Cady, Editors. 

*55. Loran, A. A. McKenzie, Editor. 

Papers of the Applied Mathematics Panel and 
Subsidiary Groups 


AMP MEMOS AND REPORTS 

*56. A Theory of Toss-Bombing, Harry Pollard, AMP Report 
146. 1R, AMG-C 411, OEMsr-1007, September 1945. 

AMP-803.5-M8 
AMP-803.5-M 12 

AMG-C WORKING PAPERS 

57. Toss-Bombing with Target Motion, Harry Pollard, 
AMG-C 293, Study 146, OEMsr-1007, Oct. 24, 1944. 

AMP-803.5-M3 

58. A Particular Method of Aiming Bombs and Rockets, 

Hassler Whitney, AMG-C 335, Study 124, OEMsr-1007, 
Dec. 15, 1944. AMP-601.2-M4 

59. A Solution of the Azimuth Problem in Toss-Bombing, 

Harry Pollard, AMG-C 438, Study 146, OEMsr-1007, 
June 9, 1945. AMP-803.5-M9 

Army , Navy , and Joint Reports 

ARMY REPORTS 

60. Operational Test of Rebecca-Eureka for Control of Tactical 
Bombardment Aircraft, AAF Board, Project 2567B413.44, 
Oct. 7, 1944. 

61. Operational Test and Determination of Tactical Require- 
ment for BUPS ( AN/UPN-1 , -2), AAF Board, Project 
3216B413.44, Nov. 17, 1944. 

62. Standard Operating Procedure for Bombing with 
AN/APS-15 and AN/APQ-13, AAF Board, Project 
3800B413.44, Feb. 23, 1945. 

63. Test of Rebecca-Eureka ( AN/APN-2 , AN/PPN-1 ) and 
SCR-729, SCR-695 for Front-Line Demarcation, AAF 
Board, Project 3898B413.44, Dec. 11, 1944. 


b References to specific sections and chapters of the RL Technical Series may be incorrect because of revisions made after 
this bibliography was printed. 


312 


BIBLIOGRAPHY 


64. Test of Portable “X” Band Beacons ( AN/UPN-3 , -4) and 
H2X Radar Front-Line Demarcation , AAF Board, Proj- 
ect 4010B413.44, May 9, 1944. 

65. Instructor’s Guide for Radar Trainer ( AN /APQ-7-T1 ), 
Headquarters, Army Air Forces Manual 81, May 1945. 

66. Radar Bomb Score Analysis , Bernard Vinograde, Report 

23, U. S. Army Air Forces, Second Air Force, Operations 
and Training Division, Colorado Springs, Colo., Sept. 
14, 1945. Div. 14-329. 15-M4 

67. Statistical Reports on Radar Bombing, C. E. Duncan and 
Gorman E. Turner, U. S. Army Air Forces, Second Air 
Force, Colorado Springs, Colo., May 26, 1945. 

Div. 14-329. 15-M2 

68. The Development of the Ground Radar Program in IX 
Tactical Air Command, Operational Research Section 
Report 64, Oct. 27, 1944. 

*69. Close Support SCR-584 Operations in the IX Tactical Air 
Command, Operational Research Section Report 66, 
Nov. 26, 1944. 

70. Utility in a Tactical Air Command of SCR-584 as Modified 
for Close Cooperation , Operational Research Section Re- 
port 86, June 13, 1945. 

71. Low Altitude Blind Bombing by Radar, Operations Anal- 
ysis Section, Report 13, OAD-43, June 20, 1944. 

*72. Method of Photo Bomb Scoring for the Radar Bombing 
Technique, Plans and Analysis Section, Department of 
Training and Operations, Victorville Army Air Field, 
Victorville, Calif., Apr. 1, 1945. Div. 14-329.151-MI 

73. Special Eagle Project Accomplished at Grand Island, 

Army Air Field, Grand Island, Nebraska: Part I, Report; 
Part II, Bombing Team Training Procedures, D. E. Baker, 
Report 353.41E1, U. S. Army Air Forces, 17th Bombard- 
ment Operational Training Wing, Sioux City, Iowa, May 
5, 1945. Div. 14-329. 131-MI 

74. Radar Bombing Accuracy, Statistical Control Office, 

U. S. Army Air Forces, Victorville Army Air Field, 
Victorville, Calif., July 1945. Div. 14-329.15-M3 

75. Final Report on Extended Training Experiment, R. C. 
Davidson, Headquarters, Victorville Army Air Field, 
July 11, 1945. 

76. “LAB vs Jap Shipping,” RADAR No. 6, Nov. 15, 1944. 
76a. Ibid. , p. 3. 

77. “H2X Synchronous Bombing/’ RADAR No. 7, Jan. 1, 
1945. 

77a. Ibid., p. 3. 

78. “Nosmo? Visar?,” RADAR No. 8, Feb. 20, 1945. 

78a. Ibid., p. 24. 

79. “Shoran: 5 Year Old Newcomer,” RADAR No. 8, Feb. 
20, 1945. 

79a. Ibid., p. 16. 

80. “Just How Accurate is H2X Bombing?,” RADAR No. 9, 
Apr. 30, 1945. 

80a. Ibid., p. 43. 

81. “Radar-Computer Combinations for Pacific Bomb- 
ing,” RADAR No. 10, June 30, 1945. 

81a. Ibid., p. 12. 

82. “Shoran’s Record in Europe,” RADAR No. 10, June 30, 
1945. 

82a. Ibid., p. 47. 

83. “The Eagle Goes to War” (Part 2), RADAR No. 11, 
Sept. 10, 1945. 

84. “Pulse Doppler,” RADAR No. 11, Sept. 10, 1945. 

84a. Ibid., p. 20. a _ 


85. Handbook of Instructions with Parts Catalog, BOMBING 
POSITION COMPUTER, Type K-l ( Electronic ), Army 
Air Forces, CO. 11-30-6-M, Jan. 20, 1945. 

86. Interim Instructions for Indicator Equipment AN/APQ- 
5B , Dayton Signal Corps Publication Agency, CO. 08-30 
APQ 5-4, Sept. 7, 1944. 

87. Final Report on Training Method and Evaluation of the 

Acceleration Integrator Bomb Release, Stuart H. Getz, 
U. S. Army Air Force Training Research and Liaison 
Section, Williams Field, Chandler, Ariz., Project 7-2-45, 
Nov. 27, 1945. Div. 14-41 1.4-MI 

88. Shoran Photo Reconnaissance Mapping and Charting 
Tests, Stanley D. Pierce, ATSC, Wright Field, Ohio, 
Sept. 3, 1945. 

89. A Simplified Method of Sighting and Releasing Bombs 
from Airplanes, H. S. Morton, U. S. Army, Ordnance De- 
partment, Report 1, Feb. 13, 1943. Div. 14-329.143-MI 

90. Preliminary Mathematical Analysis of Toss-Bombing, 

H. S. Morton, U. S. Army, Ordnance Department, Re- 
port 2, Feb. 13, 1943. Div. 14-329. 17-M2 

91. General Technique for Bombing Stationary or Moving Tar- 

gets, H. S. Morton, U. S. Army, Ordnance Department Re- 
port 3, Feb. 22, 1943. Div. 14-329. 17-MI 

92. Mathematical Study of the Timing Function of the Ac- 
celeration Integrator , H. S. Morton, U. S. Army, Ord- 
nance Department, Report 4, Feb. 25, 1943. 

Div. 14-329. 18-MI 

93. Conference on Electronic Aids to the Operations of Troop 
Carrier Command and the Airborne Army, Special Com- 
mittee on Beacons, Office of the Secretary of War 
(Minutes of Meetings), Feb. 6, 7, 8, 1945. 

94. Plotting Equipment RC-294 , Technical Manual TM11- 
1220, War Department, Feb. 17, 1945. 

Div. 14-265.3-M2 

NAVY REPORTS 

95. Preliminary Reports of Tests of Toss-Bombing with the 
Modified AIBR Equipment and AYF Altimeter, Report on 
Project TED PTR-31A11, U. S. Naval Air Station, 
Patuxent River, Md., Mar. 22, 1944. 

96. Evaluation of Toss-Bombing Equipment, Report on Proj- 
ect TED PTR-31A11, U. S. Naval Air Station, Pa- 
tuxent River, Md., Jan. 19, 1945. 

97. Test on Toss-Bombing Equipment in the F4U Airplane, 
Report on Project TED PTR-31A52, U. S. Naval Air 
Station, Patuxent River, Md., Apr. 27, 1945. 

98. Pilot’s Operating Manual for Bomb Director Mark 1 , 
Mod. 1, AN/ASG-10, CO NAVAER 08-55-501, Jan. 17, 
1945. 

JOINT REPORTS OF WAR AND NAVY 
DEPARTMENTS 

*99. Handbook of Maintenance Instructions for AN/APA-5 
and AN / APA-5A Indicator Equipment , War and Navy 
Departments, CO-AN 16-30 APA 5-2-M, May 1, 1945. 

American Manufacturing Companies 

BELL TELEPHONE LABORATORIES \_BTL \ ] 

*100. Notes on Close Support Plotting Board D- 17 0500, BTL, 
Western Electric Co., Nov. 22, 1944. Div. 14-265. 1-M2 


;estricted 


BIBLIOGRAPHY 


313 


101. Supplementary Notes on Close Support Plotting Board 
D-170500, Part One, Electronic Pilot D-170823 ( Training 
Equipment ), BTL, Western Electric Co., Jan. 20, 1945. 

Div. 14-265. 1-M4 

102. Supplementary Notes on Close Support Plotting Board 

D-170500, Part Two, Null Voltage Test Set KS-9470, 
BTL, Jan. 20, 1945. Div. 14-265. 1-M5 

103. Supplementary Notes on Close Support Plotting Board 
D-170500, Part Three, Revised Spare Parts and Equip- 
ment List, BTL, Western Electric Co., Mar. 13, 1945. 

Div. 14-265. 1-M6 

104. Supplementary Notes on Close Support Plotting Board 

D-170500, Part Four, Variable Scale Factor Conversion 
Kit D- 17 1032 ( Modification to Close Support Plotting 
Board D-171020 ), BTL, Western Electric Co., June 20, 
1945. Div. 14-265. 1-M7 

105. Temporary Information, Close Support Plotting Board 
D-171020, BTL, Western Electric Co., June 27, 1945. 

Div. 14-265. 1-M8 

LUKAS-HAROLD NAVAL ORDNANCE PLANT 

106. Toss-Bombing, L. T. E. Thompson, RTR 16, The Lukas- 

Harold Naval Ordnance Plant, Indianapolis, Ind., June 
30, 1945. Div. 14-329. 17-M 19 


RCA LABORATORIES 

107. Shoran, a New Type of Radar System for High-Precision 

Position-Finding in Aerial Navigation, AN/APN-3, 
AN /CPN-2, Serial No. 58, RCA Laboratories, In- 
dustrial Service Division, Radio Corporation of America, 
W-535-sc-671, July 1944. Div. 14-327.2-MI 

BRITISH REPORTS 

108. Oboe — Electric Mouse , F. E. Jones, Telecommunica- 
tions Research Establishment, Malvern, Eng., TRE Re- 
port T-1500, WA-829-3a, May 11, 1942. 

Div. 14-329. 132-MI 

109. Errors Arising From the Use of a Repeater Aircraft with 
Oboe, Mathematics Group, Telecommunications Re- 
search Establishment, Malvern, Eng., TRE Report 
T-1448, WA-813-8N, Apr. 30, 1943. Div. 14-329. 132-M2 

*110. Oboe — How It Works, J. E. N. Hooper, Telecommunica- 
tions Research Establishment, Malvern, Eng., TRE 
Report 4/M-101, WA-986-2a, July 2, 1943. 

Div. 14-329. 132-M3 

MISCELLANEOUS BOOKS 

*111. Air Navigation, P. V. H. Weems, McGraw-Hill, 1943. 


PART III 


Chapters 14-16 


NDRC Division Iff. Reports 

RL REPORTS 

1. Airborne 3 cm Radar Equipment for A I and ASV Ap- 
plications, N. F. Ramsey, RL Report 27 (D-4), May 22, 
1942. Div. 14-326. 1-M2 

*2. Report on Night Fighter Pursuits, Hubert M. James, RL 
Report 117 (V-4S), June 13, 1941. Div. 14-326. 1-MI 
*3. A New Approach Procedure for Night Fighting, Hubert M. 
James, RL Report 178 (43-5 Abridged), June 30, 1942. 

Div. 14-326. 1-M3 

Div. 14-326. 1-M4 

*4. A Statistical Treatment of Certain Phases of Aerial Com- 
bat, Hubert M. James, RL Report 181 (43-8), July 30, 
1942. Div. 14-600-MI 

5. Effect of Routine Evasive Action on the Calculated Ap- 

proach Procedure, Hubert M. James, RL Report 187 
(43-15), Dec. 16, 1942. Div. 14-326. 1-M5 

6. Use of Range Clock in Night Fighting with A I Equipment, 
Hubert M. James, RL Report 204 (43-32), Apr. 28, 1943. 

Div. 14-326.1-M6 

7. AIA Indicators, L. A. Haworth, RL Report 311 (62-4), 

Nov. 16, 1942. Div. 14-242. 12-M3 

8. Search Scans and System Performance, Willoughby M. 
Cady, RL Report 407, Aug. 9, 1943. 

Div. 14-234.3 1-MI 


9. Altitude Return in the AN/APS-6, Eugene W. Cowan, 
Jr., RL Report 706, Mar. 26, 1945. Div. 14-234.325-M2 

10. Sea-Return Effects and Their Elimination in the 
AN/APS-6, E. W. Cowan, RL Report 707, OEMsr-262, 
Project NS-171, June 11, 1945. Div. 14-234.325-M3 

11. Test of Type C Data Presentation with the Spiral Scan 
Aircraft Interception System, Eugene W. Cowan, Jr., RL 
Report 767, OEMsr-262, July 8, 1945. Div. 14-326-M5 

12. Mechanical Resonant Scanner, D. B. Nichinson, R. Sher, 

and C. Schultz, RL Report 782, OEMsr-262, Mar. 13, 
1946. Div. 1 4-234. 322-M4 

*13. Design Considerations for An Improved Interception ( AI ) 
Radar: The AN/APS-21 System, Randal McG. Robert- 
son, RL Report 868, OEMsr-262, Dec. 15, 1945. 

Div. 14-323.2-M11 

14. Boresighting the AN/APG-15 Antenna Assembly , C. F. 
Chubb, RL Report 1009, OEMsr-262, Apr. 23, 1946. 

Div. 14-234. 122-M 18 

15. Preliminary Instruction Manual AN / APG-15B, J. Vance 

Holdam, RL Report M-215, OEMsr-262, Project SC-69, 
June 1, 1945. Div. 14-323. 12-M8 

16. Flight Tests on A PS-6 A, R. M. Alexander, RL Report 

S-25, OEMsr-262, Projects NA-125 and NS-171, Nov. 
30, 1944. Div. 14-326. 1-M7 

17. Anti-Clutter Circuits for AEW, Vernal Josephson, Leon 
B. Linford, J. L. Lawson, and C. H. Palmer, Jr., RL 
Report S-52, OEMsr-262, Project NA-178, Aug. 1, 1945. 

Div. 14-321. 14-M13 


% 



314 


BIBLIOGRAPHY 


RL INTERNAL REPORTS AND INFORMAL 
MATERIAL 

18. Some Factors Governing the Range of A I Sets , T. W. 
Bonner, RL Internal Report Group 91, May 3, 1944. 

Div. 14-325. 1-M4 

19. Reduction of the Effect of Ground Clutter on the SCR-720 , 

Eugene W. Cowan, Jr., RL Internal Report Group 91, 
June 7, 1945. Div. 14-263. 1-M3 


RL TECHNICAL SERIES' 

*20. Cathode-Ray Tube Display , George Valley, Editor. 
20a. Ibid., Chap. 14. 

*21. Radar System Engineering, L. N. Ridenour, Editor. 
21a. Ibid., Chap. 2. 


PART IV 

Chapters 17-22 


NDRC Division 3 

CIT REPORTS 

1. Method of Computing Trajectories and Sighting Tables 
for Forward Firing Aircraft Rockets, L. Blitzer and L. 
Davis, Jr., JDC 17, OEMsr-418, CIT, Feb. 20, 1944. 

Div. 14-323.6-MI 

2. The CIT Aircraft Rocket Sight Type 2, H. W. Babcock, 
OSRD Report 2263, JNC 23, OEMsr-418, CIT, Oct. 15, 

1944. Div. 1 4-323. 6-M2 

3. Forward Firing of Rockets from P-5 IK Aircraft, OSRD 
Report 2347, JNC 26, OEMsr-418, CIT, Feb. 10, 1945. 

Div. 14-323. 6-M4 

4. Principles of Rocket Firing from Aircraft Illustrated, 
OSRD Report 2428, JNC 30, OEMsr-418, CIT, Apr. 2, 

1945. Div. 14-323.6-M5 

5. Trajectories of Aircraft Rockets 3." 5 and 5."0 , OSRD Re- 
port 2225, UBC 27, OEMsr-418, CIT, Sept. 25, 1944. 

Div. 1 4-323. 61-MI 

6. Trajectories of 11.75 Aircraft Rockets, OSRD Report 
2290, UBC 30, OEMsr-418, CIT, Nov. 17, 1944. 

Div. 14-323.61-M2 

7. Sight Settings for 2.25" , 3.5", and 5.0 " Aircraft Rockets 
Used on SB2C-1, SB2C-1C, SB2C-3 and SB2C-4, OSRD 
2275, UNC8, OEMsr-418, CIT, Nov. 23, 1944. 

Div. 14-323.6-M3 

NDRC Division 7 

OSRD REPORTS 

8. Vector Gunsights and Assessing Cameras, Division 7 Re- 
port to the Services 96, OSRD Report 5646, Project 
NO-152, Sept. 30, 1945. 

FRANKLIN INSTITUTE REPORTS 

9. The Rasp Rocket Sight Model III, Franklin Institute 
Report 330-1706-206, OEMsr, Feb. 1, 1945. 

10. Grasp Sight for Forward Firing Aircraft Rockets, Model 1, 
Franklin Institute Report 330-1706-211, OSRD 4991, 
Project NO-216, Apr. 19, 1945. 

SECTION 7.2 REPORTS 

11. Air Mass Coordinate Method of Aerial Gunnery Assess- 
ment, E. G. Pickels, Section 7.2 NDRC, Feb. 15, 1945. 

AMP-503. 3-M5 

c References to specific sections and chapters 
this bibliography was printed. 


12. Summary Technical Report, Airborne Fire Control, Part 
III, “Aerial Gunnery,” J. B. Russell, Div. 7, Vol. 3. 

13. Summary Technical Report, Airborne Fire Control, Part 
I, “Aiming Controls in Aerial Ordnance,” G. A. Phil- 
brick, Div. 7, Vol. 3. 

UNIVERSITY OF TEXAS REPORTS 

14. Development of a Machine for Testing Airborne Gunnery 
Systems, Lucien La Coste, University of Texas, Dec. 15, 

1943. 

15. Tracking with the G. E. Ring Sight, G. E. Pedestal Sight, 
and Sperry Turret with Fixed Sight , Lloyd A. Jeffress, 
War Research Laboratory, University of Texas, June 24, 

1944. 

16. A Device for Computing a Correction in the Kinematic 

Lead Computation of Lead Computing Sights, Part I, 
Lucien La Coste, War Research Laboratory, University 
of Texas, Feb. 26, 1945. AMP-503.6-M37 

17. Tests of Mark 18 Sight, Lloyd A. Jeffress, University of 
Texas, Mar. 14, 1945. 

17a. Ibid., p. 14. 

18. Preliminary Report on the Bias Error of the Mark 18 
Sight, Lawrence E. Brown, University of Texas, June 4, 

1945. 

NDRC Division 14 

DIVISION 14 REGULAR REPORTS 

19. The Sperry Stabilized Aircraft Gun Laying System Inter- 

mediate Phase, Sperry Gyroscope Co., Inc., Report 289, 
OEMsr-642, May 1944. Div. 14-323. 13-M2 

RADIATION LABORATORY REPORTS 

20. Rapid Scanning, High Resolution, Antennas — Prelimi- 

nary Report, Charles V. Robinson, RL Report 265 (54- 
17), Feb. 15, 1943. Div. 14-234.322-MI 

21. Antenna Feeds for %" Stub-Supported Coaxial Line, 

Stanley Breen and Ralph Hiatt, RL Report 271 (54-23), 
June 21, 1943. Div. 14-234.21-M5 

22. Conical Scanning, R. S. Phillips, RL Report 367 (81-1), 

Aug. 4, 1942. Div. 14-234.321-M2 

23. Lighthouse Tube Transmitter -Receiver LHTR Mk I, 
H. L. Schultz, RL Report 429, Sept. 10, 1943. 

Div. 14-310.212-M2 

be incorrect because of revisions made after 


of the RL Technical Series may 


BIBLIOGRAPHY 


315 


24. G. E. — GL2CJ+0 Taut-Grid Lighthouse Tubes, P. A. 
Cole, RL Report 600, OEMsr-262, Nov. 14, 1944. 

Div. 14-232.2-M4 

25. Overland Falcon , E. H. B. Bartelink, RL Report 647, 
OEMsr-262, Projects AC-81 and NA-205, Feb. 7, 1944. 

Div. 14-323.2-M8 

26. AN / APG-21 {Terry), E. A. Slusser, RL Report 794, 
OEMsr-262, Projects AC-81 and NA-205, Aug. 25, 1945. 

Div. 14-323.3-M9 

27. A Low Power X-Band RF Gas Switch, T. S. Ke and L. D. 
Smullin, RL Report 841, OEMsr-262, Oct. 19, 1945. 

Div. 1 4-233. 424-M2 

28. Range and Tracking Accuracy of AN/APG-15, C. T. 
Burner, RL Report 875, Mar. 22, 1946. 

Div. 14-323. 12-M9 

29. AN / APG-13B Vulture Rocket Computer, T. E. Lawrence, 

RL Report 909, OEMsr-262, Projects AC-235.01 and 
NA-205, Jan. 23, 1946. Div. 14-323.6-M8 

30. Angular Alignment of Radar Antennas, E. M. Bailey, Jr., 
RL Report 950, OEMsr-262, Mar. 29, 1946. 

Div. 14-234.6-M11 

31. Note on a Low Power S-Band Gas Switch, T. S. Ke, RL 
Report 979, OEMsr-262, Dec. 10, 1945. 

Div. 1 4-233. 424-M3 

32. Boresighting the AN/APG-15 Antenna Assembly, C. F. 
Chubb, RL Report 1009, OEMsr-262, Apr. 23, 1946. 

Div. 14-234. 122-M18 
*33. AN/APG-13 System Manual, H. T. Hodges and E. A. 
Slusser, RL Report M-152C, OEMsr-262, Project SC- 
103, Aug. 8, 1944. Div. 14-323.2-M6 

34. Preliminary Technical Manual for Falcon Trainer 
AN / APG-13-T1, Wilfred Roth, W. N. Simonds, Jr., 
Editor, RL Report M-182, OEMsr-262, Projects SC-57, 
SC-62.12, and NA-186,Oct. 20, 1944. Div. 14-411. 122-MI 
*35. Preliminary Instruction Manual AN / APG-15B, J. Yance 
Holdam, Jr., RL Report M-215, OEMsr-262, Project 
SC-69, June 1, 1945. Div. 14-323.12-M8 

36. Preliminary Instructions for Radar Set , AN / APG-13B , 

M. Boas, RL Report M-246, OEMsr-262, Project AC- 
235.01, Sept. 15, 1945. Div. 14-323.2-M10 

36a. Ibid., Sec. VI. 

37. General Description, Special Installation Requirements 

and Mounting Dimensions of AN /A PG-o { ARO ) Air- 
borne Range Only , T. E. Lawrence, RL Report S-6, 
OEMsr-262, Jan. 31, 1944. Div. 14-323.11-MI 

RL FILM 

38. AN / APG-13B, Sound Film, Radiation Laboratory. 

39. Overland Falcon, Silent Film, Radiation Laboratory. 

RL INTERNAL REPORTS AND INFORMAL 
MATERIAL 

40. Lecture Outline for Course on AN/APG-13 Falcon, H. H. 
Wheaton, RL Internal Report Group 64.2, July 25, 1944. 

Div. 14-323.2-M5 

41. Project Falcon {Air-to-Surface Vessel Radar Range for 
75-mm Cannon in B-25), Initial Report, Carl F. J. 
Overhage, RL Internal Report Group 91, Dec. 15, 1943. 

Div. 1 4-323. 2-MI 


42. AN/APG-13 {Project Falcon ) (First Supplementary Re- 

port), Carl F. J. Overhage, RL Internal Report Group 
91, Jan. 24, 1944. Div. 14-323.2-M2 

43. Frequency Pulling of ARO IfiJj Lighthouse Cavities, E. A. 
Slusser, RL Internal Report Group 91, Feb. 14, 1944. 

Div. 14-241 .42-M3 

44. The Effects of Cavity Bias on the ARO Cavity Operated by 

the ARO Modulator, E. A. Slusser, RL Internal Report 
Group 91, Apr. 27, 1944. Div. 14-323. 11-M2 

45. Errors of Optical Range Determination, Paul R. Halmos, 
RL Internal Report Group 91.5, July 20, 1945. 

Div. 14-243-M3 

46. The Solenoid Camera Drive, C. W. Mautz, RL Internal 
Report Group 91.5, Oct. 10, 1945. Div. 14-264. 1-M4 

47. Calculation of Errors in Conical Scanning, G. L. Systems 
Arising from Detuning When the Transmitter is il Pulled" 
During the Rotation, L. Jackson Laslett, RL Internal 
Report Group 94, February-March 1943. 

Div. 14-234.321-M3 

48. Tests of AGG-1 Installed in Tail of B2/D Airplane, L. 
Jackson Laslett, Charles F. West, and George W. 
Curran, RL Internal Report Group 94, Mar. 5, 1943. 

Div. 1 4-323. 13-MI 

49. Summary of Work on Propeller Modulation at the Radia- 
tion Laboratory, Julian M. Sturtevant, RL Internal Re- 
port Group 103, Mar. 21, 1944. Div. 14-324. 1-MI 

50. AN / APG-13B Vulture, E. H. B. Bartelink, RL Informal 

Report. Div. 14-323.2-M12 

51. Letters Discussing APS-25 and Related Ideas — ASH, 
E. H. B. Bartelink and others, RL. 

Div. 14-323.2-M14 

52. Final Report AN /APG-21 {Terry), E. A. Slusser, RL 

Informal Report. Div. 14-323.2-M13 

53. History of AN / APG-5, ARO, A. F. Sise and B. P. 
Bogert, RL Informal Report, Jan. 15, 1946. 

Div. 14-323.1 1-M4 

RL TECHNICAL SERIES d 

*54. Electronic Instruments, B. Chance, Editor. 

*55. Microwave Antenna Theory and Design, H. M. James, 
Editor. 

55a. Ibid., Chap. 16. 

*56. Theory of Servomechanisms, H. M. James, Editor. 

56a. Ibid., Chap. 19. 

*57. Radar Beacons, A. Roberts, M. D. O’Day, Editors. 

57a. Ibid., Chap. 1. 

57b. Ibid., Sec. 8.3. 

Papers of the Applied Mathematics Panel and 
Subsidiary Groups 

AMP MEMOS AND REPORTS 

*58. An Introduction to Analytical Principles of Lead Com- 
puting Sights, S. MacLane, AMP Memo 55. 1M, AMG-C 
137, OEMsr-1007, Mar. 27, 1944. AMP-503.6-M17 

58a. Ibid., p. 15. 

59. Bias Errors of the K-3 and K-12 Sights, Irving Kaplansky 
and Mae Reiner, AMP Memo 104.3M, AMG-C 368, 
OEMsr-1007, May 1945. AMP-502.1 1-M13 


d References to specific sections and chapters of the RL Technical Series may be incorrect because of revisions made after 
this bibliography was printed. 


RESTRICTED 


316 


BIBLIOGRAPHY 


60. The Combination of a Random and a Systematic Error , 
A. Sard, AMP Memo 104.4M, AMG-C 299R, OEMsr- 
1007, September 1945. AMP-502. 141-M9 

*61. A Proposed Vector-Sight for Airborne Fire Control, 
Magnus R. Hestenes, AMP Memo 104. 5M, AMG-C 
492, OEMsr-1007, October 1945. AMP-502.1-M33 

*62. An Analytic Study of the Performance of Airborne Gun 
Sights, Donald P. Ling, AMP Report 104. 1R, AMG-C 
440, OEMsr-1007, June 1945. AMP-502.1-M25 

*63. Deflection Formulas for Airborne Fire Control, Magnus R. 
Hestenes, AMP Report 104.2R, AMG-C 247R, OEMsr- 
1007, October 1945. AMP-503.3-M8 

63a. Ibid., p. 133. 

64. The Theory of an Electro-Magnetically Controlled Hooke’ s 
Joint Gyroscope, Donald P. Ling, AMP Report 104. 3R, 
AMG-C 262, OEMsr-1007, October 1945. 

AMP-502. 1-M34 

65. Aerial Gunnery and Gyro Sights: A Manual for Gunners, 

Donald P. Ling, AMP Report 104.4R, AMG-C 489, 
OEMsr-1007, October 1945. AMP-502.12-M22 

66. The Optical System of the Mark 18 ( K15 ) Gyro Gunsight 
with an Appendix of the Tracing Rays through a Thick 
Lens or System of Lenses , L. C. Hutchinson, AMP Re- 
port 104.5R, AMG-C 261, OEMsr-1007, October 1945. 

AMP-502. 12-M23 

67. Average Percentages of Own Speed Deflection, Dan Zelinsky 

and M. J. Lewis, AMP Memo 119.1M, AMG-C 354, 
OEMsr-1007, January 1945. AMP-503.3-M4 

68. Position Firing Rules for the A-26, Dan Zelinsky, AMP 

Memo 119.2M, AMG-C 331 (Rev.), OEMsr-1007, 
March 1945. AMP-503.4-M7 

69. What Percent of Own Speed Deflection? , Gustav A. Hed- 

lund, AMP Report 119.1R, AMG-C 270, OEMsr-1007, 
November 1944. AMP-503.3-M3 

*70. The Sighting Problem for Airborne Rockets, Hassler 
Whitney, AMP Report 124.1R, AMG-C 444, OEMsr- 
1007, October 1945. AMP-601. 2-M21 

*71. Angular Rate Methods in Rocket Sighting, Irving Kaplan-, 
sky, AMP Report 124.2R, AMG-C 493, OEMsr-10070 
October 1945. AMP-601. 2-M20 

71a. Ibid., Chap. Y. 

71b. Ibid., Chap. VI. 

72. Ballistic Calibration of Radar Range Aids to Airborne 
Cannon Fire, R. M. Thrall and G. W. Mackey, AMP 
Report 130.1R, AMG-C 498, OEMsr-1007, October 
1945. AMP-503. 1-M13 

72a. Ibid., Table II. 

*73. Camera Evaluation of Bomber Gunsights, A. A. Albert, 
AMP Report 142. 1R, AMG-N 50, OEMsr-1379, July 
1945. AMP-502. 14-M9 

74. General Principles of the General Electric CFC Computer; 

Models 2CHICI and 2CHIDI , Magnus R. Hestenes, 
Daniel C. Lewis, and F. J. Murray, AMP Memo 143. 1M 
AMG-C 346, September 1945. AMP-503.5-M14 

75. Gyroscopes of the General Electric CFC Computer in the 

B-29 Airplane, Magnus R. Hestenes, Daniel C. Lewis, 
and F. J. Murray, AMP Memo 143.2M, AMG-C 
345, OEMsr-1007, September 1945. AMP-503.5-M12 

76. The Axis Converter and the Potentiometer Resolver in 
General Electric B-29 Computer, Magnus R. Hestenes, 


Daniel C. Lewis, and F. J. Murray, AMP Memo 143.3M, 
AMG-C 346.1, OEMsr-1007, September 1945. 

AMP-503. 5-M 13 

*77. Equations for Aerodynamic Lead Pursuit Courses, Leon 
W. Cohen, AMP Report 153. 1R, AMG-C 316, OEMsr- 
1007, July 1945. AMP-503.7-M12 

*78. Aerodynamic Lead Pursuit Courses, Leon W. Cohen, AMP 
Report 153.2R, AMG-C 443, OEMsr-1007, July 1945. 

AMP-503. 7-M 11 

*79. The Time of Flight Setting of a Lead Computing Sight, 
Irving Kaplansky, AMP Memo 155. 1M, AMG-C 351, 
OEMsr-1007, March 1945. AMP-503.6-M38 

80. Simple Rules for Support Fire with a Vector Sight, Level 
and Related Attacks, Charles Nichols, AMP Memo 
157. 1M, AMG-N 37, OEMsr-1379, May 1945. 

AMP-503.4-M8 

81. Camera Assessment of Fighter Plane Gun Sights, H. L. 

Garabedian, AMP Report 160. 1R, AMG-N 52, OEMsr- 
1379, October 1945. AMP-502.14-M14 

82. Results of a Recomputation of Sight Evaluation Test Data % 

Wallace Givens, AMP Memo 166. 1M, AMG-N 79, 
OEMsr-1379, September 1945. AMP-502.141-M10 

*83. Airborne Tracking and Ranging Errors, A. Sard, AMP 
Memo 166.2M, AMG-C 488, OEMsr-1007, October 
1945. AMP-503.2-M27 

84. The Analysis of Records of Gun Errors, R. F. Bennett and 
A. Sard, AMP Report 166.1R, AMG-C 370, SRG 440, 
October 1945. 

85. Frangible Bullets and Aerial Gunnery, Gustav A. Hed- 

lund, AMP Memo 167.1M, AMG-C 442, OEMsr-1007, 
July 1945. AMP-504.52-M5 

86. The Optimum Dispersion for the Nose Guns of a B-29, 
Arthur Sard and R. L. Swain, AMP Memo 188.1M, 
AMG-C 490, OEMsr-1007, October 1945. 

AMP-504.21-M15 

87. Measurement of Angle of Attack and Skid in Rocket Fire 
Problems, H. L. Garabedian, AMP Report 191. 1R, 
AMG-N 61, OEMsr-1379, October 1945. 

AMP-502. 14-M13 

88. Analytical Studies in Aerial Combat, E. W. Paxson, 
Part A, Volume 2, AMP STR. 

88a. Ibid., Chap. 8. 

88b. Ibid., Chap. 4. 

88c. Ibid., Chap. 5. 

88d. Ibid., Chap. 8, Sec. 2. 

*89. Tracking and Fire Control Problem, Hassler Whitney, 
AMP Note 21, AMG-C 441, OEMsr-1007, September 
1945. AMP-503.2-M24 

*90. Aerial Gunnery Problems, Saunders MacLane, AMP 
Note 22, AMG-C 491, October 1945. 

91. A Manual for the Use of Gnomonic Charts, A. A. Albert, 
AMP Note 23, AMG-N 62, OEMsr-1379, October 1945. 

AMP-503. 1-M 14 

*92. Notes on the Assessment of a Bomber’s Defensive Fire, W. 
Weaver, AMP Working Paper 1, October 1944. 

AMP-504. 1-M 15 

93. A Method of Analyzing Aerial Gun Camera Film Based 
on Use of Distant Reference Point , AMP Report UNM/ 
W-32, OEMsr-1390, Project AC-92, Dept, of Physics, 
University of New Mexico, Apr. 14, 1945. 

AMP-502. 14-M7 


RESTRICTED 



BIBLIOGRAPHY 


317 


94. Report on APG-5 Test with Mark 18 ( K-15 ) Sight in B-17 
Aircraft, No. 42-4339155, AMP Report UNM/W-34, 
OEMsr-1390, Project AC-92, Dept, of Physics, Univer- 
sity of New Mexico, Apr. 30, 1945. AMP-502.12-M18 


AMG-C WORKING PAPERS 

95. A General Principle Regarding the Design of Instruments 

with Special Reference to Lead Computing Sights, G. 
Piranian, AMG-C 239, Study 155, OEMsr-1007, Aug. 
8, 1944. AMP-503.6-M32 

96. Graphical Summaries of the Mechanism Errors of Various 

Airborne Fire Control Systems, AMG-C 300, Study 104, 
OEMsr-1007, December 1944. AM P-503. 1-M9 

97. A Model Calibration for the Mark 21 and Mark 23, 

Irving Kaplansky, AMG-C 320, Study 155, OEMsr-1007, 
Nov. 28, 1944. ‘ AMP-502.1 3-M6 

98. Notes on the Tracking Problem for Fighter Planes , Hassler 

Whitney, AMG-C 329, Study 164, OEMsr-1007, Dec. 
13, 1944. AMP-503.2-M20 

99. The Eglin Field Conference on a Figure of Merit for 

Gunsights, AMG-C 342, Study 104, OEMsr-1007, Dec. 
27, 1944. AMP-502. 1-M19 

100. A Figure of Merit for Sight-Turret Performance, Arthur 

Sard, AMG-C 343, Study 104, OEMsr-1007, Dec. 29, 
1944. AMP-502. 1-M20 

101. A Rocket Sight Called Pars, Hassler Whitney, AMG-C 
359, Study 164, OEMsr-1007, Jan. 27, 1945. 

AMP-601. 2-M5 

102. The Irrelevance of Angle of Attack for the Mark 23, Irving 

Kaplansky, AMG-C 386, Study 155, OEMsr-1007, 
Mar. 16, 1945. AMP-502. 13-M 14 

103. Calibration for Straight Line and Pursuit Courses, Irving 

Kaplansky, AMG-C 390, Study 155, OEMsr-1007, 
Mar. 21, 1945. AMP-503.7-M10 

104. “ Peanut ” as a Rocket Sight, Irving Kaplansky, AMG-C 
399, Study 124, OEMsr-1007, Apr. 13, 1945. 

AMP-601 .2-M10 

105. A Proposed Device for Fighter Film Assessment, Samuel 

Eilenberg and John H. Lewis, AMG-C 404, Study 104, 
Apr. 20, 1945. AMP-504.51-M13 

106. Local Stabilization of Coordinates, P. A. Smith, AMG-C 

417, Study 166, May 16, 1945. AMP-503.1-M11 

107. Remarks on Skid in a Fighter Plane, Hassler Whitney, 
AMG-C 418. Study 164, OEMsr-1007, May 18, 1945. 

AMP-601 .2-M13 

108. The Sperry S-8B Stabilized Sight, S. Eilenberg, AMG-C 

420, Study 104, May 22, 1945. AMP-502.13-M17 

109. Experimental Verification of Optimum Percentages of Own 
Speed Lead, Arthur Sard and Dan Zelinsky, AMG-C 439, 
Study 119, OEMsr-1007, June 11, 1945. AMP-503.3-M6 

110. The Use of Range Rate in a Fighter Gun Sight, Irving 

Kaplansky, AMG-C 450, Study 164, OEMsr-1007, 
June 30, 1945. AMP-503.2-M26 

111. Preliminary Analysis of the S-4 Sight, E. R. Lorch and 

D. Zelinsky, AMG-C 451, Study 104, OEMsr-1007, 
July 7, 1945. AMP-502.13-M19 

112. The Lead for Aircraft Rockets on a Firing Course, Hassler 

Whitney, AMG-C 462, Study 124, OEMsr-1007, July 
18, 1945. AMP-601 .2-M19 


113. Diary of A. Sard. July 7, 1945, The Hypothetical Target 
in Fighter Camera Assessment, Arthur Sard, AMG-C 469, 
Study 166, OEMsr-1007, July 21, 1945. 

AMP-502. 14-M10 

114. Details on Fairchild’s Stabilization and Universal Com- 

puter, E. R. Lorch and D. Zelinsky, AMG-C 469, Study 
104, OEMsr-1007, July 23, 1945. AMP-503.5-M11 

115. Application of the Air Mass Coordinate Method to Aerial 

Gunnery Assessment, P. A. Smith, AMG-C 471, Study 
187, OEMsr-1007, October 1945. AMP-502.1-M29 

116. The Mark 23 Sight. A Brief Analytical Presentation of the 
Principles of the Pilot’s Gyro Gun and Rocket Sight, 
Irving Kaplansky and Donald P. Ling, AMG-C 482, 
Study 155, OEMsr-1007, Aug. 14, 1945. 

AMP-502. 13-M20 

*117. AMG-C Bibliography. 

118. A Modified Computation Procedure for Camera Bomber 
Sight Assessment, A. A. Albert, AMG-N 47, Study 166, 
May 2, 1945. 

119. Determination of Directions in Space by Photographs of 
Two or Three Fixed Points, A. A. Albert, AMG-N 56, 
Study 142, OEMsr-1379, June 20, 1945. 

AMP-502. 14-M8 


Army, Navy, and Joint Reports 

ARMY REPORTS 

120. Supplementary Test of Radar Range Finder in B-25-H 
Airplane; Training Manual for Use of the AN / APG-13 
in B-25-H Airplane, AAFB Project (M-3) 26a, Apr. 25, 

1944. 

*121. Test of Evaluation of Aerial Guns and Gunsight, Report on 
AAFB Project No. F3270, Test 2-44-22, Eglin Field, 
Jan. 29, 1945. 

122. Test of AN/APG-13A, AAFB Project 3904C413.44, 
Jan. 4, 1945. 

123. Test of AN /APG-13 A Gunsight ( Falcon ) as a Low Alti- 
tude Bomb Sight, AAFB Project 4073C413.44, Feb. 12, 

1945. 

124. Operational Suitability of AN / APG-13B, AAFB Report, 
Eglin Field Report on Test 2-45-39, October 1945. 

125. Fighter Gunnery, Rocket Firing, Dive Bombing, AAF 
Fighter Gunnery School, Foster Field, Texas, Air Forces 
Manual 64, May 1, 1945. 

126. Falcon (AN /APG-13) Project in South West Pacific , 
Fifth Air Force Service Command, June 15, 1944. 

127. Gun Climb, Harmonization and Bullet Pattern, Opera- 

tional Research Section, Eighth Air Force, Nov. 12, 
194 4 . Div. 14-323. 1-M2 

128. Bullet Dispersion for B-17 and B-2/+ Aircraft, Research 
Bulletin 123, AAF Central School for Flexible Gunnery, 
Laredo, Tex., Oct. 1, 1944. 

129. Bullet Dispersion for B-29 Aircraft, Research Bulletin 
143, Project 208, Research Division, AAF Central 
School for Flexible Gunnery, Laredo, Tex., Aug. 1945. 

130. Photometeor onic Method for Bomb Ballistics and for 
Measurement of Flight Performance of Aircraft, Aberdeen 
Proving Ground Report 279, Sept. 26, 1945. 


318 


BIBLIOGRAPHY 


NAVY REPORTS 

131. Handbook of Operating Instructions for Radar Set 
AN/APG-13A , Navy AN08-30APG13-2, July 19, 1944. 

132. Handbook Operating Instructions AN/APG-5, Navy, 
AN-16-30APG5-2, Mar. 20, 1945. 

133. Boresighting and Effective Angle-of-Attack Data for Air- 
craft , Bureau of Ordnance OP 1296, Dec. 29, 1944. 

American Manufacturing Companies 

GALVIN MANUFACTURING CORPORATION 

134. Tests Conducted at Northwestern University under Galvin 

Sub-contract F-olfi from September 1, 1943, to November 
1, 1943, Nov. 15, 1943. Div. 14-503-M3 

135. Project Report on Development and Production Samples of 

APG Series Radar Equipment, Contract OEMsr-972 
(, Symbol 2610) and DIC-60452, OEMsr-262, Galvin 
Manufacturing Corp., Division 14, NDRC Report 569, 
June 30, 1945. Div. 14-323. 1-M3 

*136 .Handbook of Maintenance Instructions for AN/APG-5, 
Galvin Manufacturing Corp., Attachment to Division 
14, NDRC, Report 569, OEMsr-972. Div. 14-323.11-M6 

GENERAL ELECTRIC CORPORATION 

137. APG-1 Tracking and Firing Tests — Test Request 68, 
Flight Test Project Aviation Division, GE Data Folder 
72649, Jan. 15, 1945. Div. 14-244.1-M2 


LUKAS-HAROLD NAVAL ORDNANCE PLANT 

138. Lukas-Harold Reports on Various Rocket Sights, LHL 
(Re 4d-25), 2-45 G, 3-45f, 3-45z, 4-45-S, second supple- 
ment to RTR 10, June 30, 1945. Div. 14-323. 6-M7 


British Reports 

139. The Stability of Blind Firing Systems, C. W. Gilbert, 

GRU/M.8 (Gunnery Research Unit, Exeter, Eng.), 
Mar. 14, 1944. Div. 14-323.12-M3 

140. Notes on the Stability of “ Village Inn” Mark I and on 
Means of Improving It, A. A. Hall, Armament Dept., 
RAE, Fire Control Section, F. C. Memo 97, June 1944. 

Div. 14-323.31-M2 

141. Notes on a Method of Obtaining Operational Stability in 
A.G.L. Mark I G.G.S. Systems, A. A. Hall, Armament 
Dept., RAE, Fire Control Section, F. C. Memo 98, 
Report Arm. S. 1031/J/AAH/135, July 1944. 

Div. 14-323.31-M3 


Miscellaneous Books 

142. The Design of Experiments, R. A. Fisher, Third Edition, 
Oliver and Boyd, Ltd., Edinburgh, 1942. 

143. Statistical Methods for Research Workers, R. A. Fisher, 
Third Edition, Oliver and Boyd, Ltd., Edinburgh, 1944. 


PART V 


Chapters 

NDRC Division 1/+ 


RL REPORTS 

1 . Photography of Successive Pulse Reflections from a Moving 
Target, J. L. Lawson, RL Report 348 (64-5), June 12, 
1942. Div. 14-264-MI 

*2. The Detection of Moving Targets Among Ground Clutter 
by Coherent Pulse Methods , R. A. McConnell, RL Report 
480, Dec. 14, 1943. Div. 14-263.1-MI 

*3. The Observation of R-F Phase in Pulse Radar, R. A. 
McConnell, A. G. Emslie, RL Report 481, Dec. 23, 1943. 

Div. 14-264-M4 

4. Elimination of Ground Clutter, E. C. Pollard, RL Report 

526, Mar. 13, 1944. Div. 14-263.1-M2 

5. Pulse Doppler for Detection of Moving Ground Targets, 
R. F. Thomson, RL Report 553, Apr. 21, 1944. 

Div. 14-310.13-M2 

*6. A Moving Target Selector Using Deflection Modulation on 
a Storage Mosaic, R. A. McConnell, A. G. Emslie, and 
F. Cunningham, RL Report 562, June 6, 1944. 

Div. 14-263-MI 

7. Precision Z Sweep Generator, R. A. McConnell, RL Re- 
port 563, May 23, 1944. Div. 14-242.5-M3 

*8. Effects of Clutter Fluctuation in MTI, H. Goldstein, RL 
Report 700, Dec. 27, 1945. Div. 14-263. 1-M4 


23-24 

9. A Theory of a Supersonic Delay Line, V. Hughes, RL 
Report 733, Sept. 15, 1945. Div. 14-21 1.2-M3 

10. The Storage of Video Signals on Simple Mosaics by 
Electron Beams at High Secondary Emission Ratios, 
R. A. McConnell, RL Report 743, Feb. 18, 1946. 

Div. 1 4-234.33-M5 

11. An Experimental MTI System, R. A. McConnell, RL 

Report 744, Apr. 18, 1946. Div. 14-263-M14 

12. A Measurement of Supersonic Velocities in Mercury at 
15 MC/S per Second as a Function of Temperature, R. 
Jacobson, RL Report 745, Sept. 20, 1945. 

Div. 14-252.2-M3 

13. MTI for MEW, G. Nonnemaker, RL Report 752, May 

24, 1945. Div. 14-263-M5 

14. Fluctuations in the Return Signals from Random Scatterers 
( window , rain, sea echo), A. Siegert and Francks W. 
Martin, RL Report 773, Jan. 24, 1946. 

Div. 14-122.1 14-M2 

15. Multiple Reflection Delay Tank, H. Shapiro and G. 
Donald Forbes, RL Report 791, Aug. 11, 1945. 

Div. 1 4-263. 2-MI 

16. On the Theory and Performance of Liquid Delay Lines, 

A. B. Huntington, A. G. Emslie, and A. E. Benfield, 
RL Report 792, Aug. 31, 1945. Div. 14-21 1.2-M4 

17. A Method of Rating the Stability of Local Oscillators for 
MTI, S. Roberts, RL Report 819, Oct. 16, 1945. 

Div. 14-263-M9 


% t RESTRICTED 


BIBLIOGRAPHY 


319 


18. Absorption Coefficient of Styraloy Line, Howard Row- 
land, RL Report 827, Mar. 4, 1946. 

Div. 14-233.413-M10 

19. Supersonic Delay Lines , H. Shapiro, G. Donald Forbes, 
RL Report 850, Mar. 15, 1946. Div. 14-21 1.2-M6 

*20. Detector Cancellation Error as a Function of Carrier 
Frequency, W. Selove, RL Report 859, Oct. 31, 1945. 

Div. 14-124-M3 

21. An Application of Pulse Technique to the Measurement of 

Supersonic Waves in Liquids, M. Cefola, M. C. Droz, 
S. Frankel, E. M. Jones, G. Maslach, and C. E. Teeter, 
RL Report 963, Mar. 30, 1946. Div. 14-423-M3 

22. A Method of Compensating the Frequency Dependence of 
Attenuation in a Supersonic Delay Line, Robert D. 
Arnold, RL Report 965, Dec. 27, 1945. 

Div. 14-21 1.2-M5 

*23. A Moving COHO Conversion Unit, Y. A. Olsen, RL Re- 
port 975, Apr. 3, 1946. Div. 14-263-M13 

24. 584-MTI MC-642, M. Starr, RL .Report 985. 

*25. The Firefly Moving Vehicle Detector (AN /APS-27), 
E. M. Lyman and H. L. Schultz, RL Report 994, Feb. 
18, 1946. Div. 14-321. 13-MI 

*26. Notes on MTI Receivers, W. Selove, RL Report 1010, 
Mar. 25, 1946. Div. 14-263-M11 

*27. Dynamic Range Compression for MTI, W. Selove, RL 
Report 1016, Mar. 15, 1946. Div. 14-263-M10 

*28. An Experimental S-Band AMTI System, H. G. Voorhies, 
RL Report 1018, Mar. 29, 1946. Div. 14-263-M12 
*29. Butterfly Moving Vehicle Detector AN / APS-26, C. R. 
Ahern, RL Report 1021, Feb. 15, 1946. 

Div. 14-263. 1-M5 

30. RF Phasing of Magnetrons, R. C. Fletcher, J. E. Evans, 
arid F. F. Rieke, RL Report 1051, Feb. 6, 1946. 

Div. 1 4-232. 19-M 15 

*31. Moving Target Indication on MEW, A. G. Emslie, RL 
Report 1080, Feb. 19, 1946. Div. 14-263-M2 

*32. Preliminary Technical Manual SCR 584- MTI Modifica- 
tion Kit No. MC-642- AS and Fan Beam Search Antenna, 
H. Bekkar, RL Report M-218, June 1, 1945. 

Div. 14-263-M6 


33. Specifications for 15 MC Supersonic Crystals for Crystal 

Cartridges Types 3 and 7B, P. Rosenberg, RL Report 
S-35, Jan. 22, 1945. Div. 14-422. 1-M2 

34. Velocity of Propagation of 15 MC Ultrasonic Pulses in 
Liquids , P. Rosenberg, RL Report S-56, Nov. 5, 1945. 

Div. 14-423-M2 


RL INTERNAL REPORTS AND INFORMAL 
MATERIAL 

35. Proposed Sea-echo Measurements with Airborne-MTI 

Plane, H. Goldstein, RL Internal Report Group 42, Aug. 
14, 1945. Div. 14-122. 112-M2 

36. Continuously Indicating Audio Spectroscope for C. W. 

Systems, R. H. Dicke, RL Internal Report Group 61, 
Dec. 30, 1943. Div. 14-251.5-M3 

37. Pulse Doppler with References to Ground Speed Indica- 
tion, D. Sayre, RL Internal Report Group 63, May 29, 

1944. Div. 14-124-M2 

38. Bibliography of R. L. Literature on MTI as of Feb. 6, 

1945, R. A. McConnell, RL Internal Report Group 65, 

Feb. 6, 1945. Div. 14-263-M2 

39. Some MTI Nomenclature in Use at R. L. Laboratory, 

M.I.T., R. A. McConnell, RL Internal Report Group 
65, May 2, 1945. Div. 14-263-M4 

40. Estimated, Limitations of Kit MC-642 (MTI for SCR- 
584), F. Cunningham and R. A. McConnell, RL Internal 
Report Group 65, June 18, 1945. Div. 14-263-M6 

41. Coherent Integration, A. G. Emslie, RL Internal Report 

Group 103, May 16, 1944. Div. 14-125-M8 

42. MTI Using Coherent IF, A. G. Emslie, RL Group Re- 
port Group 104, Aug. 22, 1945. Div. 14-263-M7 


RL TECHNICAL SERIES 

*43. Radar System Engineering, L. N. Ridenour, Editor, 
Chap. 15, “Orientation of MTI Problems.” 



OSRD APPOINTEES 


Division 14 

Chief 

Alfred L. Loomis 


Secretaries 

Edward L. Bowles 

John R. Loofbourow 

John G. Trump 

Technical Aides 

John C. Batchelor 

Edward H. Cutler 

John L. Danforth 

Henry 0. Eversole, Jr. 

{Administrative Aide) 

Frank D. Lewis 

John R. Loofbourow 
Nora M. Mohler 
John G. Trump 
Fletcher G. Watson 

Members 

R. R. Beal 

W. R. G. Baker 

Edward L. Bowles 

Ralph Bown 

Lee A. DuBridge 

Melville Eastham 

Ray C. Ellis 

John A. Hutcheson 

Loren F. Jones 

M ervin J. Kelly 
Ernest 0. Lawrence 
George Metcalf 

I. I. Rabi 

C. Guy Suits 
Frederick E. Terman 
Alan T. Waterman 
Warren Weaver 

H. Hugh Willis 


Section 14.1 Radar Model Shop 

(. Discontinued April 1944) 

Chief 

Melville Eastham 


Members 

Lee A. DuBridge 

Eli C. Hutchinson 

Frederick E. Terman 

A. H. Poillon 

G. Guy Suits 


Section 14.2 Navigation 

( Discontinued April 1944) 

Chief 

Melville Eastham 


Members 

Ralph Bown 

J. Curry Street 

320 RESTRICTED J* 

Donald Fink 


INDEX 


The subject indexes of all STR volumes are combined in a master index printed in a separate volume. For access to 
the index volume consult the Army or Navy agency listed on the reverse of the half-title page. 


A scope indicator, 31, 116, 284,. 289 
A1 gun sight, 257 
Absorbing screens, radar, 29 
Absorption bands, atmospheric, 55 
Absorption cone for radar testing, 186 
Acceleration integrator bomb release 
(AIBR), 145 

Aerial combat, use of radar; see AGL 
radar, AGS radar, ARO radar 
Aerial reconnaissance by beacon navi- 
gation, 126 

Aerodynamic lead pursuit course, 248 
AEW radar (airborne early warning), 
33, 53, 300 

AGL radar (airborne gun-laying), 198, 
200-218 
accuracy, 212 
antenna, 204, 213 
anti-jamming considerations, 210 
automatic following, 201 
automatic frequency control, 210 
automatic gain control, 210, 215 
automatic range tracking, 209, 215 
boresighting, 200, 210 
computers, 201, 212 
defensive systems, 202, 217 
design factors, 204-212 
functions, 200 
IFF, 202, 210 
indicators, 210, 215 
jet-propelled aircraft, 205 
offensive systems, 202, 218 
operation, 200 
photographic tests, 212 
recommendations for future equip- 
ment, 212 

servomechanism, 207 
specifications, 202 
testing, 262-275 
wavelength, 204 

AGL radar systems; AN/APG-1, 200- 
218 

AN/APG-2, 200-218 
AN/APG-3, 200-218 
AN/APG-1 6, 200-218 
AN/APG-1 9, 200, 202, 205, 206 
Black Maria, 212 
Sperry AGL-2, 206 
AGS radar (airborne gun sight), 219- 
225, 248-261 
antenna, 220 
application, 219 
component packaging, 219 
computer, 220, 248-261 
identification, 223 
indicator, 219 
photographic tests, 266 
sights for fixed guns, 220 
stability, 222 
testing, 262-275 

AGS radar systems; AGL-T, 222 
AN/APG-1 5, 224 


AI radar (aircraft interception), 175- 
193 

angular coverage, 179 
antenna beamwidth, 178 
firing methods, 191 
frame speed, 179 
future trends, 175 
ground and sea clutter, 180, 192 
ground-controlled interception ( GCI ), 
175, 176 
gun-laying, 177 

independent interception, 175, 176 
indicator display, 182 
infrared techniques, 176 
interception techniques, 188 
maintenance, 25, 185 
maximum range, 178 
navigational function, 180 
pre-radar AI techniques, 188 
probability of interception, 190-191 
radar contact probability, 189 
radome design, 186 
range clock, 192 
range performance, 178 
scans, 181, 186 
sensitivity, 178 

ship-controlled interception, 175 
special test equipment, 185 
tactics, 188-193 
windscreen projection, 187 
AI radar systems for controlled inter- 
ception; AIA, 184 
AN/APG-3, 183 
AN/APG-16, 183 
AN/APS-6, 183-188 
AN/APS-6A, 184-186 
AN/APS-19, 183-186 
SCR-540, 183 

AI radar systems for independent in- 
terception; AN/APG-1, 183 
AN/APG-2, 183 
AN/APS-4, 184 
AN/APS-21, 183-186 
SCR-520, 183 
SCR-720, 183 

Aided-tracking box for the “Vulture,” 
234 

Air mass coordinate test method for 
bomber gunnery, 272 
Air navigation; see Navigation, aerial 
Air speed meter for GPI, 98 
Air to ground combat, radar for, 252 
Air to ground fire-control systems, tests 
of, 273 

Airborne cannon, 256 
Airborne Early Warning svstem 
(AEW), 32, 53, 300 
Airborne fire control radar systems; see 
AGL radar, AGS radar, ARO 
radar 

Airborne gunsights; see AGL radar, 
AGS radar, ARO radar 

RESTRICT ED v jP 


Airborne magnetometer, 11 
Airborne moving target indication 
(AMTI), 176, 181, 279-302 
cancellation unit, 283-285 
doppler principle, 279 
limits of detectability, 284, 300, 302 
presentation methods, 283, 287, 290 
recommendations, 301 
theory, 279 

Airborne moving target indicator sys- 
tems; Butterfly (AN/APS-26), 
284, 287 

Cadillac (AN/APS-20), 300-302 
Firefly (AN/APS-27), 290 
Airborne radar mapping systems; see 
Radar mapping systems 
Airborne radar ranging principles, 59 
Airborne radar systems for bombing; 
see Radar bombing 

Airborne range-only radar systems; see 
ARO radar systems 
Aircraft interception, pre-radar tech- 
niques, 188 

Aircraft interception, radar; see AI 
radar 

Aircraft velocity vector, 37 
Airspeed determination for GPI, 98 
Altimeter, Kollsman aneroid, 146 
Altitude circle on radar screen, 48 
Amplifier, low input capacity, 295 
Amplitude cancellation unit for AMTI, 
293 

AMTI; see Airborne moving target in- 
dicator 

AN/APA-5 (LAB bombsight), 78, 290 
AN/APA-5 range computer, 80 
AN/APA-46 (Nosmo), 74-76 
AN/APA-47 (Visar), 74-76 
AN/APG-1 (AI and AGL system), 
183, 191, 198-207, 210, 212, 
261 

AN/APG-2 (AI and AGL system), 
183, 202-205, 261 

AN/APG-3 (AGL system), 183, 202- 
206, 261 

AN/APG-4 (Sniffer), 139 
AN/APG-5 (ARO system), 226, 229, 
262 

AN/APG-13 (Falcon), 198, 199 
AN/APG-13A (Falcon), 199, 226 
AN/APG-13B (Vulture), 199, 218, 226, 
256 

AN/APG-15 (AGS system), 219, 220, 
261 

AN/APG-1 5B (AGS system), 223, 
264 

AN/APG-16 (AGL system), 183, 202- 
205, 207, 210 

AN/APG-19 (AGL system), 202, 205, 
207 

AN/APG-21 (Terry), 199 
AN /APG-22 (AGS system), 149 


321 


322 


INDEX 


AN/APG-24 (AGS system), 149 
AN/APN-1 (altimeter), 139 
AN/APQ-5 (LAB), 78, 290 
AN/APQ-7 (Eagle), 50, 58, 63, 67, 73 
AN/APQ-10, Radar bombing com- 
puter, 66 

AN/APQ-13 (H2X), 50, 59, 67 
AN/APQ-13, Mod. II, (MX-344), 82 
AN/APS-3 (low altitude radar bomb- 
ing system), 58 

AN /APS-4 ( ASV and AI radar), 177 
AN /APS-6 ( AI system), 183, 191 
AN/APS-6A (AI system,) 184 
AN/APS-15 (H2X system), 16, 50, 54, 

58, 64, 67, 71, 100 

AN /APS-15A dial computer, 71 
AN /APS-15A modulator, 287 
AN /APS-15A range unit, 61 
AN /APS-15BM radar system, 50 
AN/APS-19 (AI system), 183-186 
AN/APS-20 (Cadillac) radar system, 

59, 300-302 

AN/APS-21 (AI system), 183-186 
AN/APS-26 (Butterfly), 287 
AN/APS-27 (Firefly), 290 
AN/APS-30-series ASV systems, 22 
AN/APS-33 (ASV system), 30, 50, 58, 
59 

AN/APS-34 (ASV system), 50, 59 
AN/APS-35 (ASV system), 50 
AN/APX-15 (Ella), 224 
AN /ARA-1 7 release point indicator, 159 
AN /ASG-10 toss-bombing system, 153 
Aneroid altimeter, 146 
Angular-travel lead, 258 
Antennas, radar; beamwidth, 178 
dipole arrays, 13, 28 
gyro control, 19 
hemispherical search, 216 
low-frequency ASV patterns, 13 
polystyrene rod type, 246 
radiation patterns, 52, 55 
rotating disk, 213, 220 
rotating paraboloid, 213, 220, 236 
search antenna, 213 
size and weight, 204 
small antennas, 246 
stabilization, 18, 19 
test equipment, 64 
tracking antenna, 213 
Yagi array, 13 

Anti-jamming provisions, 118, 210 
Anti-radar devices, 11, 118, 209, 210 
Anti-submarine tactics with ASV, 32 
AN/UPN-1 portable beacons, 114 
Applied Mathematics Panel, NDRC, 
197, 235, 249-263 
Arma resolver, 90 

ARO radar (airborne range-only), 226- 
247 

advantages, 226 
antenna recommendation, 246 
computers, 248-261 
future developments, 244-247 
Lighthouse Transmitter-Receiver 
unit (LHTR), 226 
tactical application, 226 
testing, 262-275 


ARO radar systems; AN/APG-5, 148, 
226, 229 

AN /APG-13A (Falcon), 226, 231 
AN/APG-13B (Vulture), 226, 234- 
241 

AN/APG-21 (Terry), 226, 234, 241 
ASB radar (ASV system), 14 
ASB-3 (ASV system), 58 
A-scope indicator, 31, 116, 284, 289 
ASE radar (ASV system), 13 
ASV radar (air to surface vessel), 9-33 
antenna patterns, 2, 13 
antenna stabilization, 18, 19 
azimuth resolution, 17, 50 
beacon equipment, 22 
design, 17-24 
design criteria, 9 
identification of friend or foe, 33 
indicator, 13, 31 
limitations, 30, 33 
maintenance, 25-30 
manuals of instruction, 29 
maximum range, 30 
means of detection, 13-16 
methods of attack, 32 
military iises, 10-12 
navigational feature, 9, 22 
operator fatigue and safety, 22, 31 
panel layout of set, 21 
patrol methods, 31 
peacetime uses, 9 
performance, 25-35 
pulse duration, 17, 18 
recommendations, 32 
reliability, 21 
scanning rate, 20 
sea return, 20 
test equipment, 29 
Vixen attachment, 11 
ASV radar systems; AN /APS-4, 177 
AN/APS-30, 20-24 
AN/APS-3 1, 23 
AN/APS-3B, 23, 30 
ASB radar, 14 
ASE radar, 13 
ASVC radar, 13 
ASVC radar, (ASV system), 13 
Attack methods with radar, 32 
Audio frequency phase shifter, 283 
Audio indication for radar system, 287 
Automatic controls for radar systems, 
58 

Automatic following radar; see AGL 
radar 

Automatic frequency control and oper- 
ator comfort (radar), 58 
Automatic frequencv control with 
AGL radar, 206, 210 
Automatic gun laying; see AGL radar 
Automatic range tracking with AGL 
radar, 209 

Azimuth control with MX-344 com- 
puter, 83 

Azimuth errors, effect on GPI, 100 
Azimuth resolution in ASV radar, 17. 
50 



RESTRICTED 



B scope presentation, 15, 57, 77, 182, 
210 

B-29 pedestal sight, 251 
B-29 standard computer, 258, 260 
Ballistic calibration of radar for cannon 
fire, 252 

Ballistic cams for Vulture, 235 
Ballistic computation with MX-344 
computer, 85 

Bandwidth, optimum radar receiver, 56 
“Barber Pole,” skid indicator, 273 
Battle of the Bulge, 279 
Beacon bombing, 104-130 
advantages and disadvantages, 104, 
127 

airborne beacons for extending range, 
127 

beacon offset bombing, 105, 121 
GPI computer design, 130 
methods of increasing range, 127 
methods of simplifying operation, 
129 

types of systems, 104 
Universal computer design, 130 
Beacon bombing systems; beacon offset 
systems, 105, 121 
Gee, 39, 123, 124 
Gee-H, 108 
Micro-H, 109-113 
Oboe bombing, 116-118 
Rebecca-Eureka equipment, 113 
Shoran, 114-116 
SS Loran, 39, 124 

Beacon navigation; aerial reconnais- 
ance and mapping, 126 
paratroop dropping, 125 
Beacon responders for AGS, 223 
Beacon test equipment, 64 
Beacons, general description, 9 
Beamwidth of several typical radar 
systems, 50 

Bell telephone laboratories, MX-344 
computer, 82 

Bias error of gun sights, 262 
Black Maria (identification system), 
212 

“Blackout unit” for Gee-H, 108 
Blimps using ASV radar, 11 
Blind bombing with Gee, 106, 123 
Blind bombing with SS Loran, 106, 
124 

Blind firing, 191 
Bomb release circle, 67 
Bomber, determining motion of, 272, 
273 

Bomber fire-control systems, tests of, 
269 

Bomber gunnery; air-mass coordinate 
test method, 272 
pre-radar, 248 
radar tracking, 251 
range-only radar, 250 
Bombing, visual, 39, 42, 45 
Bombing accuracy; methods of deter- 
mining, 155 

use of h + b technique, 69 
Bombing computers, radar; accelera- 
tion integrator, 145 


INDEX 


323 


AN /APA-5 computer, 79 
AN/APQ-5 computer, 78 
for beacon bombing, 130 
for Eagle systems (AN /APQ-7), 67,73 
for Gee-H bombing systems, 109 
for ground controlled bombing, 133 
for H-bombing systems, 108 
for MC-627 plotting table, 133 
for Nosmeagle, 74, 75 
for Nosmo (AN/APA-46), 74 
for Shoran, 114 
for toss bombing, 145 
for Visar (AN/APA-47), 74 
general discussion, 66 
Ground Position Indicator (GPI), 
90, 130 

impact predicting computers, 67-72, 
133 

LAB computers, 77, 79 
MX-344, 82-86 
non-synchronous, 66-72 
Norden bombsight, 72 
semi-synchronous, 66, 72-77 
synchronous, 66, 79-86, 133 
test equipment, 64 
Universal bombsight, 130 
Bombing procedure; computation of 
release points, 40 
from fighter planes, 252 
from low altitudes, 226 
identification of target, 39 
navigation to target, 38, 39 
radar vs. visual methods, 45-46 
steering to release point, 41 
Bombing scoring; aircraft camera 
methods, 155 
necessity, 5 

photography from ground, 156 
photo-reconnaissance, 165 
PPI photographs, 164 
use of ground radar systems, 157 
Bombing systems, radar, 37-172 
beacon bombing, 104-130 
beacon offset, 121, 122 
Gee, 123, 124 
Gee-H, 108 

ground controlled, 131-138 
Ground Position Indicator, 87-103 
H-systems, 106-116 
hyperbolic navigation, 122-125 
maintenance, 63-65 
mapping systems, 47-59 
Micro-H, 109-113 
Norden bombsight, 42 
Oboe beacon bombing, 116-129 
procedure, 37 
ranging, 59-63 
Rebecca-Eureka, 113 
Shoran, 114-116 
testing, 155-166 
training personnel, 166-172 
toss bombing, 139-154 
visual bombing, 42, 45 
Bombsights; see also Bombing com- 
puters, radar 
Norden, 42, 72, 73 
requirements, 45 

Universal bombsight (UBS), 66, 130 


Boresighting; AGL radar, 200, 211 
AI radar, 186 

British Gee navigation system, 106 
British gyro gunsight Mk II-c, 221 
British H2S radar system, 57, 67 
B-scope presentation, 15, 57, 77, 182, 
210 

BUPS radar beacon, 121 
BUPX radar beacon, 121 
Butterfly (AN/APS-26); general de- 
scription, 287-289 
performance, 290 
tactical uses, 287 


C scope presentation, 182, 210 
Cadillac system (AN/APS-20), 59, 
300-302 

Cameras for AGS system tests, 266, 272 
Cameras for bomb scoring, 155, 165 
Cancellation unit for AMTI, 283, 292 
Cannon fire control, airborne, 252 
“Carpet,” 175 

Circuits, electronic; amplifier, low input 
capacity, 295 

audio frequency phase shifter, 283 
detector balance bias (DBB) circuit, 
181 

fast time constant (FTC) coupling 
circuit, 181 

instantaneous automatic gain control 
(IAGC) circuit, 181 
jitter circuit, 108 

local oscillator, stable (STALO), 283, 
301 

phantastron delay circuit, 63 
phase detector, 208 
trigger circuit, 61 
“Close-Support” bombing, 131 
“Clutter,” 104, 180, 284 
COHO (CW oscillator), 282, 301 
Communications in ground-controlled 
bombing, 136 

Comparator for Shoran, 115 
Computers, airborne gunsighting and 
gunlaying; see Gunsights 
Computers, airborne rocket, 257 
Computers, bombing; see Bombing 
computers, radar; Bombsights 
Conical scan, 200-209, 213-215, 241, 
246 

Controls for radar systems, recom- 
mendations, 58 

Corner reflector on life rafts, 11, 12 
Cosecant-squared antenna pattern, 52, 
55 

Countermeasures, radar, 11, 118, 175, 
209,210 

Course determination by GPI, 88, 94 
Cross trail, 40 

Crystal checker for radar crystals, 29 
C-scope presentation, 182, 210 
Cursor, electronic, 58 


Dead reckoning, aerial, 37 
Deflectometer method of testing 
bomber gunnery, 270, 271 



Delay circuits, 61, 63, 69, 71 
Delay line, mercury, 292 
“Delayed” PPI, 57 

Detector balanced bias (DBB) circuit, 
181 

Dial computer (AN/APS-15A), 71 
Dipole antenna arrays, 13, 28 
Direction finding stations, ground 
based, 38 

Directional coupler (transmission line), 
28 

Director-type gun sights, 220, 258 
Dispersion error of fire-control systems, 
262 

Displays, radar; see Radar displays 
Distortions of radar maps, 47-54 
Disturbed-reticle gun sight, 221, 258 
Dome design for AI radar, 186 
Doppler effect, use in radar bombing, 
75, 279, 284 

Draper-Davis (Army A-l) sight, 241, 
246, 257, 260 

Draper-Davis S-9 computing gunsight, 
218 

Drift angle, 40 

Drum computer, (AN/APS-15), 67 
Duplexer, 14, 206 

“Eagle” radar system (AN /APQ-7) 
beamwidth, 50 
computer, 67, 73 
description, 59 
test calibrators, 63 

Echo box (radar test equipment), 29, 
289 

Electronic cursor, 58 
Enemy aircraft interception; see AI 
Radar 

Eureka beacon, 113 
Expanded gain, 215 

Falcon (AN/APG-13), 199 
Falcon (AN/APG-13A), 229, 252 
Fast time constant (FTC) circuit, 181 
Fighter aircraft; all purpose sights, 257 
gunnery radar, 251 
gunnery tests, 272 
pilot problems, 252 

Fire control radar, airborne, 197-275; 
see also AGL radar, AGS radar, 
ARO radar 

air-to-ground systems, 252, 273 
computer problems, 248-261 
system types, 197, 270 
Firefly (AN/APS-27); cancellation 
units, 292-295 

comparison with Butterfly, 290 
general description, 290 
performance, 300 
tactical uses, 290 
Firing tests with AGL radar, 212 
Fix computation with GPI, 87, 90 
Fixed guns, computer use, 220 
Flare dropping with Gee, 124 
Fluorescent maps for radar bombing, 
170 

Foot-pedal ranging, 250 


324 


INDEX 


Frequency meter, 29 
Frequency pulling of magnetron, 206, 
210 

FTC coupling circuit (Fast time 
constant), 181 
Future range, 272 

G scope presentation, 182, 225 
Gain control, automatic, 181 
Gain expander, 235 

GCI (Ground controlled interception), 
175, 188, 204; see also AI radar 
GE pantograph scanner, 206 
GE standard B-29 computer, 258, 260 
GE standard B-29 pedestal sight, 251 
Gee system; airborne instrumentation, 
~ 123 

blind bombing, 123 
comparison with H and Oboe bomb- 
ing systems, 106 

flare-dropping pathfinder operations, 
124 

ground station arrangement, 123 
limitations, 106, 124 
Gee-H bombing system, 108 -109 
Gisholt scanner balancer machine, 186 
GPI; see Ground Position Indicator 
Ground based direction finding station, 
38 

Ground clutter, 284 
Ground controlled bombing; communi- 
cation, 136 
computer, 133 

plotting table (MC-627), 133 
radar system (SCR-584), 132 
theory, 131-132 
unified control system, 136 
vectoring, 136 

Ground controlled interception (GCI), 
175, 188, 204; see also AI radar 
Ground Position Indicator, (GPI), 64, 
87-103 

air speed meter, 98 

associated equipment, 96 

computer mechanism, 90 

errors, 96-99 

functions, 87 

geometry, 87-90 

operational techniques, 101-103 

radar requirements, 99-101 

recommendations, 103 

test equipment, 64 

use with beacon bombing systems, 

130 

Ground stations for Shoran, 114 
Ground Track Aiming Point (GTAP), 

131 

Ground velocity vector, 37 
GSAP camera (N4A), 266 
G-scope presentation, 182, 225 
Gun climb, 264 
Gun roll errors, 258 
Gunlaying, radar-automatic; see AGL 
radar 

Gunnery firing rules, 248 
Gunnery scoring, aerial; AGL photo- 
scoring, 212 
AGS tests, 266 


bomber gunnery, 270 
deflectometer method, 270, 271 
fighter gunnery, 272 
necessity, 5 

photometeoronic method, 272 
Texas sight-testing machine, 262 
Tri-camera methods, 262, 270 
W-2 fighter-sight assessor, 273 
Gunsight testing, 262-275 
Gunsights, airborne; see also AGL 
radar, AGS radar, ARO radar 
AI radar, 177 
all-purpose sight, 257 
Army K-14, K-15, and K-17, 221, 
262 

B-29 pedestal sight, 251 
B-29 standard computer, 258, 260 
British gyro Gunsight Mk II-c, 221 
director-type, 220, 258 
disturbed recticle, 221, 258 
Draper-Davis A-l, 241, 246, 257, 260 
Draper-Davis S-9 computing sight, 
218 

electrical integrator type, 258, 260 

Fairchild K-8, 258 

fixed guns, 220 

general discussion, 258 

gyro, 221, 258, 260, 262 

lead computing, 258 

Mark 18, 222, 251, 258, 260, 262 

Mark 21 and Mark 23, 222 

non-gyro, 258, 260 

optical ring, 258 

PUSS (Pilots Universal Sighting 
System), 241, 257 
rate sights, 250, 258 
Sperry K-4, 258, 260 
types, 258 
vector sights, 258 

Gunsights, manually-directed with 
radar scope; see AGS radar 
Gunsights, radar range-only; see ARO 
radar 

Gyro controlled antennas, 19 
Gyro gun sights, 221, 258 
“Gyrosyn” compass, 98 

H bombing systems; advantages and 
disadvantages, 104 
Gee-H, 108-109 
general characteristics, 106 
Micro-H, 109-113 

paratroop dropping application, 114 
Rebecca-Eureka, 113 
Shoran, 114-116 

h + b technique for H2X bomb com- 
puters, 69 

H2S radar system, British, 57, 67 
H2X bombing, 164 
H2X drum computer, 67 
H2X radar systems; AN/APQ-13, 50, 
58, 59, 67 

AN/APS-15, 16, 50, 58, 64, 67, 71, 100 
AN/APS-15A, 71 
Harmonization of scanner, 186 
Hit probability nomogram for gun fire, 
262 

“Holy Moses”; see Rockets 


RESTRICTED 


Homing operations, 10, 11 
HVAR (High velocity aircraft rockets), 
254 

Hyperbolic navigation systems; ad- 
vantages and disadvantages, 
106, 122 

comparison of SS Loran and Gee, 124 

Identification as friend or foe, (IFF); 
AGL radar, 202, 212 
AGS radar, 212, 223 
AI radar, 175, 178 
ASV radar, 33 
Black Maria, 212 
propeller modulation, 223 
use of GPI, 102 

Impact-predicting bombing computers; 
dial computer, 71 
H2X drum computer, 67 
inadequacies of, 72 
modified H2X drum computer, 70 
Independent interception, 175, 176; see 
also AI radar 

Indicators, radar; see Radar displays 
Infrared techniques for AI radar, 176 
Instantaneous automatic gain control 
(IAGC) circuit, 181 
Instrument navigation, 38, 39; see also 
Navigation, aerial 

Interception of enemy aircraft; see AI 
radar 

Interference spots on PPI, 22 

Jamming, countermeasures against, 
119, 210 

Jet-propelled aircraft, difficulty of 
radar detection, 205 
Jitter circuit for Gee-H bombing 
system, 108 

K-4 sight, Sperry, 258, 260 
K-8 sight, Fairchild, 258 
K-15 gyro sight, 262 
K-band radar systems; AN / APS-34, 59 
description, 59 
limitations, 17 
rapid scan, 50, 59 
use for AGL, 205 
Kollsman aneroid altimeter, 146 

L Scope indicator, 183 
LAB computers, 77 
Lead angle, 248 
Lead computing gunsights, 258 
Lead problem in aerial combat, 248 
Lead pursuit course, 248 
Lighthouse transmitter - receiver 
(LHTR) unit, 219, 226, 236 
“Lobe-switching,” 14 
Local oscillator, stable, (“STALO”), 
283, 301 

Long wave ASV radar, 13 
Long wave beacons, 14 
Long wave radar for AI, 188 
Long wave systems for beacon offset 
bombing, 121 
Loran, 32, 39 

Low altitude bombing computers, 77 


INDEX 


325 


M scope indicator, 233, 252 
Magnetic airborne detector, 11 
Mail transference guided by ASV 
radar, 9 

Maintenance, radar; airborne bombing 
system, 63-65 
ASV radar, 25-30 
fire control equipment, 264 
instruction manuals, 29 
personnel, 29, 167 

Manually-directed radar gunsights, 
219-225; see also AGS radar 
Mapping, radar; see Radar mapping, 
Radar mapping systems 
Maps for radar use, fluorescent, 170 
Mark 18 gyro sight, 258, 262 
MC-627 plotting table, 133 
Mercury delay lines, 292-295 
MEW (Microwave Early Warning 
radar), 137 

Micro-H bombing systems; accuracy, 
166 

Mark I, 109-111 
Mark II, 111-112 
Mark III, 112, 113 

Moving target indicator; see Airborne 
moving target indicator 
MX-344 bombing computer, 82-86 


Navigation with radar, aerial; AI radar, 
180 

ASV radar, 9 
Gee, 123, 124 
general discussion, 37 
GPI, 101 

hyperbolic systems, 106, 122-124 
SS Loran, 124 
NDRC Division 7, 197 
Night fighting techniques; AGL radar, 
210 

AI radar, 175 

Noise figure in the radar receiver, 27, 56 
Non-synchronous bombing computers; 
dial computer, 71 
H2X drum computer, 67 
h + b technique, 69 
impact predicting computers, 67 
modified H2X drum computers, 70 
Norden bombsight, 42, 72, 73, 82 
Norden synchronous computer, 66, 77, 
82 

Nosmeagle bombing system, 75 
Nosmo (AN/APA-46) bombing com- 
puter, 74-77, 289 


Oboe beacon bombing systems; ad- 
vantages and disadvantages, 105 
Mark I, 116-118, 129 
Mark II, 118, 119 
Mark III, 120 

Offset bombing, 40, 66, 89, 90, 121 
Operator, radar; see Radar operator 
Optical methods for bomb scoring, 155 
Optical range determination, errors, 
267 

Optical ring gun sights, 258 


Optical-plus-radar fire control; see 
ARO radar, Gunsights 
“Own-speed lead” formula, 248 
Own-speed sight, 248, 258 

Palmer scan, 202, 207 
Panel layout of ASV radar set, 21 
Pantograph scanner, 206 
Parallax, effect on rocket sighting, 254 
Paratroop-dropping guided by radar, 
114 

Paratroop-dropping with aid of beacon 
navigation, 125 
Patrol with ASV radar, 10, 31 
Phantastron delay circuit, 63 
Phase detector, 208 
Photographic scoring for aerial bomb- 
ing, 155, 165 

Photographic scoring for aerial gunnery, 
212, 262 

Photo reconnaissance for testing bomb- 
ing accuracy, 165 

Photo reconnaissance with Shoran, 126, 
127 

Photo-theodolite method of bomb scor- 
ing, 156 

Pilot’s direction indicator (PDI), 84 
Plan position indicator (PPI); bomb 
release point, 67, 74 
distortion, 49, 72 
three-tone PPI presentation, 57 
use as a map, 47 

use with Micro-H Mark I, 110, 111 
use with Microwave ASV radar, 15 
Plotting tables for bomb scoring; MC- 
627, 157 
RC-294, 157 
RC-305, 164 

Polarization of radar beams, 20 
Polyrod antenna, 246 
Position determination of aircraft by 
GPI, 87 

Power absorption cone for AI radar 
186 

PPI; see Plan Position Indicator (PPI) 
Pulse duration in ASV radar, 17 
Pursuit course rules, 248 
PUSS (Pilots universal sighting sys- 
tem), 241, 257 

Radar beacons; see Beacons 
Radar bombing; see Bombing systems, 
radar 

Radar bombing computers; see Bomb- 
ing computers, radar 
Radar boresights, 186, 200, 21 1 
Radar contact probability, 189 
Radar development agencies, coordina- 
tion, 259-261 

Radar displays; see also Radar mapping 
A-scope, 31, 116 
B-scope, 15, 57, 77, 182, 210 
British H2S Mark IVa display, 57 
C-scope, 182, 210, 212 
delayed PPI, 57 

fixed terrain airborne display, 57 
G-scope, 182 
L-scope, 183 



M-scope, 233, 252 
PPI, 15, 47, 67, 72, 74, 110 
V-scope, 234, 236 

Radar equipment testing; see Testing, 
radar equipment 

Radar for air-to-ground combat, 252 
Radar for bomber gunnery, 250 
Radar for cannon fire, 252 
Radar for fighter gunnery, 251 
Radar for rocketry, 260 
Radar for toss bombing, 148 
Radar gunsights; see Gunsights 
Radar maintenance; see Maintenance, 
radar 

Radar mapping, 39, 47-59, 66-86 
computers, 66-86 

distortion due to airplane motion, 53 
distortion due to antenna pattern, 52 
maximum range, 54 
point of departure, 38, 39 
PPI spot size limitations, 49 
presentation other than PPI, 57 
recommendations for improvement, 
58 

resolution, 49-52 
slant range distortion, 47 
Radar mapping systems; AN/APQ-7 
(Eagle), 50, 58, 63, 66, 73 
AN/APQ-13, 50, 58 
AN/APS-3, 58 
AN/APS-15, 50, 54, 58 
AN/APS-20 (Cadillac), 58 
AN/APS-33, 50, 58 
AN/APS-34, 50, 58 
ASB-3, 58 
rapid scan, 50, 58 

Radar operator; comfort in ASV de- 
signs, 22 

cooperation with bombardier, 73 
cooperation with pilot (Falcon sys- 
tem), 231-233 
training, 166-172 

Radar systems for airborne detection 
of surface vessels; see ASV radar 
Radar systems for aircraft intercep- 
tion; see AI radar 

Radar systems for bombing; see Radar 
bombing 

Radar systems for fire control; see 
AGL radar, AGS radar, ARO 
radar 

Radar systems for moving vehicle de- 
tection; see AMTI 
Radar tracking, 202, 251 
Radar training equipment, 169 
Radar-optical fire control; see ARO 
radar, Gunsights 
Radome design for A I, 186 
Range, airborne radar; accuracy, 100 
advantages of radar ranging, 250 
AI radar, 178 
AN/APS-26, 287 
ASV radar, 25, 30 

calibration for bombing purposes, 61 
comparison with stadiametric rang- 
ing, 250 

delay circuits, 61 
errors, 61, 69, 267 


326 


INDEX 


factors affecting, 54 
for rocket firing, 256 
h b technique, 69 
maximum range, 30, 54-57 
measurements, 59 
range marks, 58, 59, 61 
resolution, 51 
stabilization, 19 
unit AN/APS-15A, 61 
unit test set, 63 

Range clock for aircraft interception, 
192 

Range-only radar systems; see ARO 
radar 

Rapid scan radar, 50, 53, 59 
Rate computation, aircraft 
with GPI, 88, 91 
with Nosmo, 76 
Rate gun sights, 258 
Rate sights with stadiametric ranging, 
250 

RC-294 plotting device; errors, 163 
use in bomb scoring, 157 
RC-305 manual plotting system for 
bomb scoring, 164 

Rebecca-Eureka beacon homing equip- 
ment, 113, 122, 125 
Receiver noise, radar, 56 
Receiver performance figure, 25 
Recommendations for future research; 
AGL radar equipment, 212 
airborne fire control radar systems, 
198, 199, 244 

airborne radar mapping systems, 58 
ASV radar, 32 

beacon bombing improvement, 127 
GPI techniques, 103 
gunlaying with AI radar, 177 
MX-344 computer, 86 
radar system controls, 58 
Recommendations for radar personnel 
training, 172 

Release point computers, 40, 89, 96 
Release point indicator AN/ARA-17, 
159 

Rescues at sea guided by ASV radar, 9 
Resolving power, radar, 15, 50, 99 
R-f test load, 29 

Rockets; High Velocity Aircraft Rock- 
ets (HVAR), 254 
Holy Moses (5" HVAR), 256 
radar ranging, 256 
sight setting, 253 
skid effect, 273 
Terry installation, 241 
Tiny Tims, 254 
trajectory drop, 255, 257 
Vulture installation, 235 
Roll-stabilization of antennas, 19 

Scanning; AGL radar, 205-207 
AI radar, 178, 181, 186 
ASV radar, 20 

conical scan, 200-209, 213-215, 241, 
246 

criteria, 16 
Gisholt balancer, 186 
harmonization, 186 


helical, 185 
Palmer scan, 202, 207 
Pantograph scanner, 206 
rapid television scan, 214 
scanner balancer, 186 
Sperry AGL-2 system, 206 
spiral, 213 
very rapid, 53, 59 
wig-wag, 185 
Schnorkel, 11, 32 

Schools for training radar operators, 
166 

Scientists in warfare, use of civilians, 
1-5 

Scoring, aerial bombing; see Bombing 
scoring 

Scoring, aerial gunnery; see Gunnery 
scoring, aerial 
SCR-520 radar, 183 
SCR-540 radar, 183 
SCR-584/M radar, 132, 157, 287 
SCR-720 radar, 183, 184 
Sea clutter, 9, 20, 104, 180, 300 
Sealed assemblies for radar systems, 24 
Search with AGL radar, 202, 207 
Search with ASV radar, 10 
Secrecy and wartime research, 2 
Semi-synchronous bombing computers, 
66, 72-79 

Sensitivity time control (STC) circuit, 
181 

Ship controlled interception (SCI), 175; 
see also AI radar 

Shipborne night fighters, use of AGS, 
219 

Shoran; airborne installations, 114 
advantages and disadvantages, 1 1 6 
comparator, 115 
computer, 64, 115 
ground stations, 114 
indicator unit, 115 
mapping procedure, 127 
photo-reconnaissance recorder, 126, 
127 

receiver, 116 
repeater schemes, 129 
test equipment, 64, 65 
transmitter, 115 
use of, 116, 126 

Sight assessing machine (University of 
Texas), 271 

Signal generator test set, 29, 64 
Skid indicator, “Barber Pole,” 273 
Sky-wave synchronized Loran; see 
SS Loran 

Slant range distortion, 47 
Slant range velocity, 78 
“Sniffer” equipment (AN /APG-4), 139 
Sono-buoys, 32 
Space modulation, 117 
Spectrum analyzer, microwave, 29 
Sperry AGL-2 scanning system, 206 
Sperry K-4 sight, 258, 260 
SS Loran; airborne instrumentation, 
125 

blind bombing, 125 
comparison with H and Oboe bomb- 
ing systems, 106 


navigational use, 124 

Stabilization of airborne radar an- 
tennas, 19 

Stable local oscillator (“STALO”), 283, 
301 

Stadiametric ranging, 250 

Standing wave meter, 29 

Storage tube, 286 

Streamlined aircraft, difficulty of radar 
detection, 205 

Submarine detection with ASV radar, 

10 , 11 

Submarine escape tactics, 32 

Surface clutter, 181 

Synchronous bombing computers; 
AN/APA-5 (LAB Mk II), 78-82 
AN/APQ-5, 78 
AN/APQ-10, 66 
AN/APQ-13 (MX 344), 82-86 
GPI, 64, 87-103 

Norden synchronous computer, 66, 
79 

UBS (Universal bombsight), 66 


Tactics with radar systems; AGL 
radar, 200 
AGS radar, 219 
AI radar, 188-193 
AMTI, 279-302 
ARO radar, 226 
ASV radar, 10, 31 
beacon bombing system, 104 
beacon navigation, non-bombing, 125 
beacon offset bombing, 122 
ground controlled bombing, 131 
toss bombing, 139, 140 
Tangential test signal, 25 
Target Discrimination, (TD), 24 
Target identification in bombing from 
aircraft, 38 

Temporal cancellation unit for AMTI, 
293 

Terry (AN/APG-21), 149, 234, 241, 
257 

Testing, radar equipment; AGL radar, 
212 

AGS radar, 262-275 
AI radar, 185 
beacon bombing, 64 
bombing computers, 64 
Butterfly, 289 
Eagle, 63 
GPI, 65 

radar maintenance, 29, 63 
range calibration, 63 
Shoran, 64 

test points on circuits, 27, 28 
toss bombing, 151 

Texas sight-testing machine, 262, 271 
Three tone PPI presentation, 57 
Time-of-flight calibration of gunsights, 
259 

Tiny Tims, 254 

Toss bombing; acceleration integrators, 
145 

barometric element, 146 
description, 139 


INDEX 


327 


from fighter planes, 252 
mathematical analysis, 140 
non-radar methods used, 145, 151 
tactics, 139, 140 
test results, 151 
use of radar, 139, 148, 153 
Tracking radar, 202, 251 
Trail of bomb, 40 
Training equipment, radar, 169 
Training of radar personnel, 166-172 
Trajectory drop of rockets, 255, 257 
Transmit-receive (TR) box, 206 
Tri-camera method for testing flexible 
gunnery, 262, 270, 271 
Trigger circuits, 61 


Universal bombsight (UBS), 66, 130 
University of Texas sight assessing ma- 
chine, 271 

V scope presentation, 234, 236 
Vector gun sights, 248, 258 
Victorville radar training experiment, 
170 

Video stretching, 24 
Visar bomb computer, 66, 74 
Visual bombing, 39, 42, 45 
Vixen attachment for ASV radars, 11 
Vulture system (AN/APG-13B); aided 
tracking, 234, 240 

arrangement and operation, 235-240 


ballistic cams, 235 
function, 234, 235 
LHTR, 236 
performance, 235 
range accuracy, 237 
rocket computer, 257 
use with cannon, 253 

W-2 fighter-sight assessor, 273 
Wig- wag scanner, 185 
Wind velocity vector, 37, 38 
“Window” (chaff), 175, 192 
Windscreen projection in AI radar, 187 

Yagi antenna system, 13 


SEP 2 6 1960 

UB * ABr W C0NG 8ESS 








declassified 

By authority Secretary of 

SEP 2 6 1960 

Defense memo 2 August 1960 
LIBRARY OF CONGRESS 






